@droftarts Thank you for your reply!
I have tried your solution and while I could not get just the A and B axes mapped, I did manage to do:
M563 P1 X0:3 Y1:4
Which made them move in sync and worked as expected.
M563 P1 X3 Y4
This did not work however. The AB axes would not move at all. Using X4 and Y5 also did not have any effect. The axes XYZABC would be numbered 012345 right?
Anyway, thanks for the reply!