Use of 1 PC and multiple accounts for Sims 4

@Jershwar  This plan should work, although there's a chance Origin might remove the packs you don't own and make your girlfriend download them again, because Origin likes to be difficult again.  What's definitely true is that you shouldn't need to have two separate installs of the base game and the packs you both own.  Origin is capable of managing one installed game shared across multiple user and Origin accounts; you're not anywhere near the first person to want this kind of setup.

If you do run into issues with Origin removing packs instead of just blocking them, and download speeds are an issue, there's a workaround that might help.  But it's not worth getting into the details unless you need it.