The way I did it was to basically mix each of the drive channels into each other. I don't know if this is the right way by any means. But basically, say you've got your left drive on the vertical axis of a stick and your right drive on the horizontal. You press the little scroll wheel in and move down to 'mix 1' and press it again. Then you should be able to select the channel you want to mix, and the channel you want to mix it into. You then just have to alter the throttle level for both (I think I set mine both to +100% or -100%) and then do the same on 'mix 2', but now you mix the channels the opposite way around. This basically means that when the regular action of the channel is added to the mixing input you've added, the output causes both drives to go forward when the stick is forward, and go in opposite directions when it's left or right.

Sorry if this explanation made no sense, I'm doing this from memory right now, and it's hard to put concisely into words. I'll do a video showing how I did it if nobody else can explain better.