Reversing the channel should do exacly the same thing as reversing the motor wires, but if ch.rev. isn't working you can do that.
I don't know how to mixing is done on your TX, but in mine i have to assign 3 pairs of master/slave channels and say their percentages, like:

Master: ch1 %:100
Slave: ch2 %:-100

I had to mess around with rev. and +/-% to get it right.

Try to reverse the motor wires, if that does the trick, you are all done.