What you need to bear in mind is that seat ECU is essentially a couple of driver IC's and some memory chips that "remember" the relative positions of seats and mirrors.They are arranged to create XYZ switching and memory, or at least it is something like that. If you open the seat ECU you will find a load of relays in there. These are what make the seats and mirrors move. I think that the mirror switches take power from the seat ecu, energise the relevant relay and move the mirror.
There is no reason why you can't rewire the mirrors to be independent, you just need to think about where you take the power from an how you will then pass the power from the switch to the correct relays. As I said before, it may well be easier to just by-pass the seat ECU by putting in the mirror controls from a non-memory seat model.