I'm with "Subculture" on this topic, rather than automatically buying an external "mixer" to install in your model, read the instruction booklet that should have come with the RC outfit. Mixing of different channels is often possible.
As most multi channel transmitters are biased towards model aircraft rather than boats, you have to familiarize yourself with the different terms, like Mode 1&2, Delta, V-tail and such like. My favorite is to use a transmitter with two dual axis sticks in MODE 1 with the "V-tail" mix selected.
This allows me to sail a twin screw model with the conventional controls of rudder on the right-hand horizontal stick movement and both motors working together on the left-hand vertical stick movement. BUT, leave the rudder alone and move the left-hand stick horizontally and you have the two motors working in opposite rotations. This can be used to rotate a stationary model or, when sailing ahead, tighten the turning circle whilst still allowing normal sailing using rudder and motor controls.
It was described a couple of times in the magazine "Model Boats", the most recent being in the "Spookoiny" plan feature (July 2023).
Glynn Guest