DJM-850 MIXER getting both tracks played in one channel

I bought a Pioneer mixer DJM- 850 and I have CDJ”s- 400
I currently have Traktor Pro. Which I use with my Traktor Z1 and my cdj 400’s and works perfectly, so I’m familiar with it Traktor pro software; but I connected my new mixer DJM- 850 with my cdj-400’s and only one of the channel works. Meaning both tracks play in the same mixer channel. I can’t get it to mixed because only one cross fader works. I cannot pre-listen to my track it comes out straight to the same channel. All channels do work , I do know that, because I already connected the cdj’s to all the channels in the mixer.
please help !

my computer is a HP envy touchsmart 15 notebook windows 8.
Thank so much in advance.