As the Sims Wikia has no page on Downtown as a type of sub-hood, I'll ask it here:
If I have the Maxis Downtown attached to a hood, and then decide to create my own Downtown, what would happen?

Back up your game first!

The only known issue with having two downtowns at once is, that the second downtown will be empty. Which is a problem if you're trying to use Tarlia's Downtown version of Belladonna Cove, but a benefit if you're using a standard or custom downtown, as you won't get two identical sets of downtownies.

If you delete a downtown and then add a new one, a second set of downtownies is generated (including the dead Tricous). These will be independent sims with full character data and new names, but the same appearance, relationships, and NPC behavior. You will not get a second Mrs. Crumplebottom, as she is a Universal NPC, not generated by adding downtown but by installing Nightlife.

@Jawusa found out how to add a 2nd downtown with sims and maybe she can link to that tutorial.

Other than this adding more downtown sub-hood is ok to do and I have done it in the past but they will be empty of sims.

I have three custom downtowns that I built myself and they all populated with downtownies - I just eventually moved graduated sims to these more urban settings.

Okay, it must just be playables that are suppressed. In which case a new custom downtown wouldn't be a problem (unless you mind duplicate divas and so on).

The dead Tricous have no graves in a downtown without the House of Fallen Trees and Gothier Green Lawns, but if that bothers you, you should be able to spawn their tombstones. And you could use an editing tool to unlink the Tricou bastards from their dead relatives and each other, or just have three families of six half-siblings running around.

So if I create the new downtown (custom), it doesn't have any downtownies? Will the Downtownies from the Maxis Downtown come to the lots in the Custom Downtown?
@Peni Griffin: you said that if I delete the Downtown and then create another there will be new downtownies. Isn't that something bad to do (deleting a sub-hood)?

Deleting a subhood is fine. Deleting sims is bad. Don't delete the original downtown until you have moved the graves at Gothier Green Lawns and the House of Fallen Trees (and anywhere else that's acquired them during gameplay) to a safe location in another part of the hood. Also, any owned businesses should be sold before deletion.

Charmful's experience would seem to indicate that I misunderstood how the downtown population glitch works. If you convert a hood populated with playables (like Bluewater Village or Belladonna Cove) to a downtown and add it to a hood with a downtown already attached, the playable sims living there will vanish. This is a bad thing.

Downtownies generate during use of a downtown, from a standard set of downtownie templates. As far as I can tell, my old downtownies are appearing right beside newly-generated ones in my new custom downtown - fortunately, I've changed the appearances of almost all my downtownies and can tell them apart! I haven't been playing the new one long and don't know if my old Master Vampires have rivals now. If you make a custom downtown, new downtownies will generate from existing templates "as needed." How your game defines "as needed" will depend, at least in part, on your particular setup and what mods you have.

When you add a second or third Downtown, there are no extra files. The second and third Downtowns use the townies/NPCs generated for the first one. Most of the downtownies/NPCs get generated when you first load the first hood though there are a few NPCs who generate as you play (an example would be the reporter from OFB).

Yes, the only reason I'm getting duplicates is that I deleted my old downtown and then added a new one.

This shouldn't be as confusing as it is, but it's difficult to talk about clearly for some reason. Also, since OFB was added people are a lot more likely to add a subhood than a second downtown, so people don't have as much direct experience with it.

I've written a step-by-step tutorial with pictures here.

As Peni Griffin said, only the first downtown will have the sims. All of the other downtowns will be empty and won't be safely cleaned; sim references can be seen in SimPE.

Before adding any custom downtowns, make sure that you take out EA's Downtown before adding any custom ones because the default downtownies could be added to your neighborhood and when you add more than one downtown, there could be duplicates of the sims with different names.

I just went in a did a test.

1. Made a test hood.
2. Add Downtown and got all the normal sims.
3. Tried to add Bayside Flats but no sims came over. (No new downtownies no new NPC no new sims at all.)
4. Deleted Bayside Flats nothing wrong happen.
5. Deleted Downtown and all the sims are still ok and the dead are still there in my test hood. (I have nounlinkondelet mod in.) This needs more testing without the mod in.
6. Add again Bayside Flats and now have all the Bayside Flats in there houses and bin.
7. Deleted Bayside Flats and the game added all the sims from the houses to the bin and they kept the money value they had, no problems found. All dead sims still in the hood.
8. Add Downtown again and again I got all the downtownies and a 2nd set of NPC and dead sims.

I will go use the hood checker on this hood and will report back about the dead sims.

So to me it seems as long as you don't deleted the org. Downtown and re-add it you are ok to just delete it and add a new populated Downtown like Bayside Flats Downtown sub-hood with no problems.

I have more testing to do but this time will do the testing without any mods in and see what happens.

On the same note, the downtownies I get show up in all three of my custom downtowns - they are not restricted to just one the same as dormies can show up across all uni lots. I didn't notice if the game populated a whole new batch per downtown or just re-used the ones from my first downtown.

I would use the method that I described in the tutorial, marka93.

As you said, you got duplicates of the downtownies which is not a good thing and did you also get duplicates of the Bayside Flats characters?

Charmful, you can easily test that. Downtownies don't come with names attached - they are randomly generated. If your downtownies always have the same names when you mouse over them, they're always the same sims.

If your second downtown generated new Tricous, you'll find two identical sets of identical dead sims with identical names and relationships in your Default family, one linked to one set of teens, and the other linked to an identical set of teens with different names.

I don't think that's a bad thing in itself, as each duplicate has a unique GUID and the game is unlikely to get confused; but it would be confusing for the player, who would be well-advised to take precautions. I can see why the game was designed in such a way as to prevent it. I'm not worrying about it in Drama Acres in any case, because that neighborhood was broken long before this happened, anyway. But my impression had been that this was an unusual case due to the deletion of the original downtown before the addition of the second one.

I don't believe that's how it's supposed to happen normally. Normally, if you add two downtowns (which exist at the same time), you should only get characters in the second one if you use Jawusa's method. Otherwise, the same downtownies will appear in both downtowns, with no duplication of character files, and no playable sims (if the second downtown was originally populated by playables) will appear.

Only after deleting the old Downtown and adding it back do I get duplicates, as long as you do not add it back you only have the org. downtownies no mater how many extra downtowns you add.

1. Add Downtown so it will gen up all downtownies and NPC.
2. Delete Downtown.
3. Add new CC Downtown with Sims.

After this you only have 1 set of downtown sims that got added when you first added Downtown, no others have gen up on me with or without mods. Just as long as you don't add back the Org. Downtown your good to go.

This is my current situation: my hood has the Maxis Downtown still with the Tricou graves.
If I move the House of Fallen Trees and Gothier Green Lawns to another location in the hood (the main hood, per example), I can safely delete it, right?
Then, I can create the new Downtown and it will have downtownies, right?

How do you plan to move these lots? If you place them in the lot bin with the graves intact, the effect is the same as if you deleted the graves, which is the same as deleting a sim.

In any case, this is separate from the issue of generating downtownies.

You already have one set of downtownies. These include the Tricous, dead and alive.

If you add a new downtown, with your existing downtown intact, these existing downtownies will continue to appear in your second downtown the same as they do in the first. The dead Tricous will stay where they are. No sims will be added or deleted.

If you then delete the first downtown, you will still have all the downtownies (as long as you moved the Tricou graves to a safe location first). No one should be added or deleted at that point.

If, however, you delete the first downtown before you add the second, you will get a second batch of downtownies - including dead Tricous - because the game will read this as adding a downtown to a neighborhood that lacked one, and will recreate all the downtownies, as brand new individuals with unique ID numbers. They will look and act exactly as your first batch of downtownies did when you first added that downtown, and will exist alongside them.

If this is still not clear, keep rephrasing your question, and we'll keep rephrasing the answer, until everybody understands everybody else. Because it does get a bit dizzying.

Thanks, Peni! I understand now what I have to do.

No because that will mean putting the lots in the Lot Bin - bad idea. The best thing to do is move them manually. Can't you move a sim in to the lot and move them to a cemetary lot in your main hood? (Make sure you go to the cemetary lot afterwards and place the graves to complete the move fully).

As long as you have the mod call "no unlink on delete" and you don't mind losing the stone (which can be regen) you can delete Downtown all you want to.
Just remember if you add Downtown again you will get all the Downtown townies and NPC again with all the dead sims.

You should not get any more Downtownies if you add a Custom Downtown unless the Custom Downtown comes with townies.

Yes, you will, Mark! The copy of Bigg City I have attached to Widespot is generating downtownies from the downtownie templates, because it was the first and remains the only downtown associated with it. I haven't looked to see whether it has dead Tricous, but I bet it does.

I just added Bayside Flats to 3 dif. test hoods as the first Downtown and the only sims that got added is the playables in the hood there are no downtownies or NPC yet, there are only the 20 or so playables sims.

Maybe more testing is needed then if some players are getting townies and other are not.

I think, this is just a theory, but I think this is how the duplicate downtownies thing works.

The game generates downtownies (inc. special townies) when you add a downtown for the first time. When adding subsequent downtowns, the game doesn't add more downtownies because it can "see" the previous downtowns and so assumes that they are there.

However when you remove a downtown, it doesn't delete the downtownies. Therefore the downtownies were actually added to the main hood (just classified as downtownies). This actually makes sense because they all go into the same characters folder, and you can't differentiate.

When you then add another downtown after deleting all downtown districts, the game assumes that this is the first and only downtown to be added to this hood, and so creates downtownies all over again.

Does that make sense? From what little I know about coding it makes sense that it would work that way.

More testing is needed.