Hey Jen --
Much love to Microsoft (my first tech job was supporting Access and the JET database engine many moons ago), but...I don't think you'll be happy with performance trying to join two Access databases. Keep in mind Tableau also leverages the JET database engine, so some of the same issues you're trying to avoid could still appear when you try to "re-join" the two split databases.
Instead, I'd suggest you leverage Tableau extracts - much faster.
- Create an extract from the the original Access database
- Right-click your extract, choose Extract from the right-click menu, and then select Add Data From File
- Make sure you change the file mask from All Files to Microsoft Access Database
- Select the other database and table(s) you need...
More rows will be added to your in-memory extract database. The drawback to this technique is that you'll have a two step refresh process (import the first database, then the second), but I'd personally pay this cost for quick response the resulting extract will give me on a daily basis.