Using both USB sticks and Rekordbox at the same time.

I have two CDJ 3000.s and a Pioneer DJM 900 NSX2 mixer.  I was creating a mix from USB sticks and realized that a few tracks I thought were on the sticks were not , they however are on my Rekordbox software. Normally the USB sticks work by setting to LINE and to use Rekordbox I would have the switch set to A in my situation. I tried to have a track play from the LINE / USB stick and the other from the A which is recordbox of my CPU. Rekordbox starts to stutter and won't let me load any track onto the CDJ. Is this the way it is supposed to be?

Where you can ONLY use either USB outputs OR Rekordbox outputs but not both at once???

Anyone?