You can initiate a manual rescan via Preferences/Media Library/Right click on folder/Rescan now. Alternatively you can set up a keyboard shortcut to do the same.I'm also pretty sure restarting foobar does the job as well.
Did you try using the shortcut? It says 'Rescan folders', which should mean it rescans all of your folders at once. If that's not the case, is there anything stopping you from storing all your music in one place?And as far as I understood you get your data from the other PC, which means you are forced to stop listening anyway if that shuts down.