Reply to Apc40 Channel Routing to External Mixer

Sorry for all the apc and mixer questions but I just wanna make sure I do this right.
To make the apc a little simpler, I would wanna route channels 1-2 to channel one on the mixer and 3-4 to channel 2 and use channels 6-8 for other functions right?
First of all does that make sense? What could i use channels 6-8 for and second how within Ableton would I map my channels 1-4 on the apc to channels 1-2 on my xone 42?