You can add Sims to your household from the Manage Households menu without having to enter the lot. But of course once you have added them you should load that lot, and lock doors (once they get inside their own apartment) to allow only them or them and the employees like a maid or repairman. You still have to play both those Sims living in two different parts of a lot. Or you could try to play one set and keep hands off the other. But I tried that and they all get annoying trying to play one sim and leave the others alone in their apartments. They all will come and go like they should. However, it can become a hassle the game puts them all out in the road if you go to build mode or switch to another house and back.
Yes, in this game you can delete any Sim and they will no longer be in the game. Ever again. Unless you put back in a copy of them from your library but of course it's not really the same Sim.
