Can you switch from one monzo account to the other within the app?
From Single to Joint, you mean? Long press the navigation bar at the bottom and you should get a pop-up
You can switch between any account which is held against your profile in app.
So if you have a Personal and a Joint Account, you can switch between them as @Hatticus has advised (in various ways depending on if you use iOS, Android, old nav, new nav)
If you mean can you switch between account(s) under your profile and the accounts under someone else’s profile - no you can’t. You’d have to log out of the Monzo ap and login using the other person’s Monzo credentials. Not the most secure approach to banking