Boat skid steering

Hi Folks

I’m working on a boat with 2 brushed motors, controlled by Cytron driver, essentially making it BrushedWithRelay + skid steering (all under Navio2).

I’m a bit lost in configuration options - skid steering requires RC1 = ThrottleLeft and RC3 = ThrottleRight, but this means servos operate as servos, not BrushedWithRelay? Or should I set RC1 = motor1 and RC3 = motor2? And why should I set RC1 and RC3 and not RC2 for example?

Sorry, a very naive question it may be, but I feel there are some aspects of Ardupilot that I completely misunderstand…