Need help with vehicle controller

I'm doing a mod that has two engines which works perfectly except for when I want to reverse the car. The main engine's gearbox shifts into reverse but the second one driving the rear wheels doesn't. The two engines oppose each other and the car ends up not moving. How can I have both transmissions shift properly? I've ended up needing to disconnect the rear motor when reversing. The files are attached so you can try for yourself.