For simplicity's sake I would use his controller with his drone. I see no advantage in doing anything otherwise, in fact it might possibly avoid firmware 'conflicts'. The latter presumes your brother'
s controller and drone have matching firmware.
I have several phones, Android and iPhone, that I use with a Mavic Mini i.e. the FLY app, and there is no problem with that expect that the flight records are 'spread out' over those phones. I do not know if the
mini 2 differs in this respect but I would be surprised if it did.
The app is able to separate drones one from another, I have flown 3 minis with one phone and each mini has its own set of records in the app