You can only live above a business owned by a household member. So, you could have 2 or more families, and 2 or more fake apartments, but all sims would have to be in the same household and you would have to rely on the old door locking separation methods to keep them to their own spaces.
Only one sim can be the actual Owner. If you want a "partner" you could try adding them as an employee and split profits for the day when you get the daily summary. I'm sure there are mods that will help with this too, like SNB. You could have business funds go into a shared account and split it from there?