That is pretty cool. Why do you need to disable Ch. 1 on the transmitter? What you need are just channel 2 and 4. To change which channel is used, you just change the port of which motor or servo you want on a different channel. To have 2 motors off say, the 2nd channel, you need to put a jumper in one of the interrupts or some other port, I don't remember though, but that should allow you to plug in a 2nd motor into motor port 8 because you have it on Ch. 2. If you had it on Ch. 1, it would have to be port 7 for the 2nd motor. They would also both run in the same direction for that, but you need to mount them a little offset.
Here's a quick drawing I made to better explain.
I hope it answers your question.