So, I'm a rotational player, and...
If you want to create a boyfriend for your Sim...
- Be in the same save file as the Sim you're creating the boyfriend for...
- Go to Manage Worlds
- Click on an empty house or apartment where you want him to live. (If you have your eye on an inhabited house for him, you can evict the current resident first!)
- There's an option (possibly under "..." that says "Create New Household To Move In."
- Create the Sim, give him whatever job you want him to have, etc. Decorate his home. All that fun stuff.
- Save.
- Go back to Manage Worlds and select the house/apartment where your original Sim lives.
The potential boyfriend should then live wherever you placed him, and you can visit him there. You can also play as the boyfriend any time you like by going to manage worlds.
If, instead, you just want to import your boyfriend's Sim into your save, and have him import your Sim into his save, and play separately without the two saves affecting each other in any way:
- Save your Sim to the library in CAS.
- Save your lot in buildbuy.
- Open his save, go to manage worlds, and place your Sim and their lot there.
- Save his Sim to the library.
- Save his lot to the library.
- Open your save, go to manage worlds, and place his Sim and their lot there.