@elastic1 > You don't really "lose" the channel when recording.
If you're looking to record your set within Traktor, see this video:
It's for the 2000, so you miss out on the setup for the mixer's soundcard, but the setup within Traktor is similar. For the 900's soundcard, within the config utility, set the CH1 output to REC OUT and then within Traktor, you select the 900nexus 1/2 as the input source for recording.
Let me know if you need more info.