Now let me see if I can relate this to paper....
I own a Tito Neri and here goes. I have independent steering independent drive and you can operate from one stick if required.
I use a FX18 transmitter with mixing on channels.
in normal mode the sticks are [right side channel 1 & 2 ]
[left side channel 3 & 4 ]
[center slide right channel 5]
[center slide left channel 6]
[back slide right channel 7]
[back slide left channel 8]
[top module switch's/pots 9--12]+
when I switch on a mixer I can use chan 1 mixed with 3 this is drive motors = both on the same stick, same speeds, same direction
I switch on another mix and chan 2 is mixed with 4 this is steering same stick same direction
If I leave the switches on the OFF position I have control of port side speed,direction and thrust on right stick and the same for the stbd side on the left stick.
now I can change over to even more variations like one stick to control both 'rudders' the same but have independent motors this lets me have the 'free stick' to control the bow thrust.
Yes I get a head ache remembering what does what and why
but what a great tug to control and use in towing.
If I was running on a limited transmitter I would have two ESC's and then link the 'rudder' control, then limit the throw to that of just above standard rudder movement this is to give control to "walk" rudder to the opposite direction of walk one motor ahead the other astern (slightly differing speeds) and without the BT the vessel will walk. also tank steering is possible thus with a little switching the steering can be switched to another requirement.
The one thing you will have to do is make sure your steering servos are as strong as you can get into the limited space to stop the prop thrust overriding the servo strength.
R,