The Fairphone 3 is a so-called A/B device.
It has 2 “slots” (A and B) it can boot an OS from, but there’s still only one place for user data, so it’s not a complete dual-boot solution.
A/B is primarily there for updating the OS in the background (on the other slot which is not being used) while you can still use the phone and then just reboot into a completely updated OS (by just switching to the updated slot on reboot).
That being said …
What does the phone do after that?