I just figured out that if I use nostr:npub1getal6ykt05fsz5nqu4uld09nfj3y3qxmv8crys4aeut53unfvlqr80nfm to log in to nostr:npub12vkcxr0luzwp8e673v29eqjhrr7p9vqq8asav85swaepclllj09sylpugg, and then set up a second account with Alby, then switching Alby accounts and refreshing the page effectively switches which Nostr account I'm logged into Primal as.

Neat!