Skid Steering Motor Test vs Manual mode directions

I’m trying to set up a boat using skid steering. I’ve set Servo1 to ThrottleLeft and Servo3 to ThrottleRight.

When setting up the motors I go to the Motor Test page and use Motor C for left and Motor D for right. My issue is when I set up the reverse parameters on the Servo Outputs page correctly so both turn clockwise (forwards), then when I use manual mode with the RC Transmitter one motor (Servo3) goes in the wrong direction.

I figure there should be a setting to change how ardupilot splits the single throttle channel from the transmitter because if I reverse the throttle input in the Radio Calibration it reverses both motor directions. Any ideas?