Is it possible to beat sync USB and PC on XDJ-RR?

I want to do a B2B with an XDJ-RR. One DJ uses a PC and the other DJ a USB stick.

Individually this works great, but trying both at the same time not so much. It is possible to have deck 1 as PC and deck 2 as USB, but the sync button (beatmatch) doesn't work when I tested this, even though it shows the BPM of both songs playing on the XDJ-RR screen. Using PC on its own the sync button works properly, and USB on its own too. Is this simply not a feature, can this be enabled in settings, or am I just doing something wrong?