Not sure on the mac...
On the PC mhere is an app where you

can tell:
Channel 1 from the mixer to the compute

: PG Dating 1 post/prefader, PGM2, etc.
Channel

2 fro Dating the mixer to the computer : PGM1 post/prefader, PGM2, etc.
So you would set:
Channel 1:bGM1 postfader
Channel 2: PGM2 postfader.
Thats it.