Skeed steer setup - astern motion

Hi everyone. How do I activate reverse motion(astern) in a boat in skid steer? Right now, I can only do a U-turn on the spot.

The ESCs need to be bidirectional.
1500us neutral, lower is reverse, higher is forwards.

I have ESC with two directions. Reverse gear is activated only on CH1(radio) when turning. On CH3(radio) in the middle position, for some reason, the forward movement is activated at 50% acceleration, and not stop as it should be.