The setup is as follows:

Four drill motors:
Back two controlled by two Electronize units
Front two controlled by Scorpion XL
Electronize and Scorpion XL RC wires going into two Y-leads and then into the receiver.

It works fine with mixing on the Scorpion (except the rear wheels aren't mixed so they run independently) so with the Scorpion set to tank steering (no mixing) it should be possible to turn on mixing in the radio and control all four wheels with one stick.

So when we set that up, it would drive correctly mixed but the two front wheels would go slower than the two back wheels. Switch it back to tank steering and the front wheels return to their normal speed. I'm not used to DX6s so I'm unfamiliar with the mixer settings; thought it might be a dual rates issue but that shouldn't be affecting the speed of the front motors when all four wheels are going in the same direction

EDIT: Tony, the way it was set up when I left was no mixing. One side of the drive was on one channel (up/down) and the other side was on another channel (left/right). When it's mixed, it's combining both channels so that up/down moves both sides of the drive instead of just one, same with left/right.