floppymanta,
I use Sabertooth ESC and they have a DIP switch setting which allows you to easily switch from 2 stick control to controlling 2 motors with one single stick. I don't know if this is possible with the brushless motors (you say drone motors) and the ESC's you are using. Also don't know if your ESC allow for motor reversing or not (not generally good on an brushless airplane or drone motor) without changing the connection polarity of the motor. The Sabertooth allows for motor reversing for brushed motors - from radio as well.
It seems like it should be possible with some kind of 2 channel mixing and a Y adapter cable to the Throttle channel (like twin engine airplane)? But cannot guarantee that setup would work for moving vehicle in reverse.
You want a single robot or surface vehicle ESC that specifically allows for controlling 2 motor, allows for motor reversing and one stick control.
Sorry can't offer much more than that....
B