First, if you are using props, take then off. They, I'd make sure both motors are working. Try switching them & make sure both work.
The basic concept, you will need 3 mixes set up 2 channels. First mix, throttle to 2nd channel 100%. I don't care what the 2nd channel is, any free channel will work. Second mix, rudder to throttle channel 100%. Third mix, rudder to 2nd channel -100%. If the rudder mix is reversed, switch the values in mix 2 & 3 OR switch the channels the ESC's are plugged into.