Thanks. At the moment I have it setup like so.

Motors/ESCs connected to CH1 and CH2 (right stick)

CH1 (up down on right stick) is mixed to CH2 so up down controls both wheels forward and back.

CH2 (left right on right stick) controls one wheel but I won't use it.

CH3 (left right on left stick) is mixed to CH1, so when I turn left the left wheel goes back, when I turn right it goes forward.

How would I use the plug and play mixer in amongst that setup to get CH3 control both wheels, turning in opposite directions?