Ofcourse you can, on the mixer, there is a knob to select the input, (USB1/USB2/RCA/Phono) or you can select i/o in Abelton preferences for example. 12 in 12 out. You have to split this 12 between with the USB selector though.
I personally think that i can only control so much at any one time, that limiting factor isnt the amount of channels you have, its the amount of hands you have, and the ergonomics. If the synth is too far away on the desk its hardly used.