Using a tank - problems with config of motors skid steer

Hi, I am using a apm 2.8 mega which does limit me to an earlier version of rover. the problem was I had a pixhawk and it was very flaky so I switched backwards to gain reliability. So looking at the docs I cannot use servo_function 73,74 for skid steering as they don’t exist in the APM version. I have managed to get the motors turning from the Radio controller but I am not clear on how to configure properly. So right now I can set it up so both motors go forward and backward from one control but of course how do you steer? When i try to set one motor on one control and the other on another it doesn’t work. Any help appreciated