Yes, you can use a mix to control 2 channels (the two that the ESCs are connected to) from one stick on just about any computerised transmitter. That's what I do using my Futaba T7C - 2 motors/ESCs on 2 separate channels, then set up a permanent (not switched) mix to make both motors run from the left stick.
It can also be helpful with the steering to use more mixes to add a bit of right motor to left rudder, and left motor to right rudder. Just set up 2 mixes to add, say, 30% of the motor channel to the rudder. Mixes like this are proportional, so more rudder stick means more motor, with full rudder giving 30% motor in this case. More modern transmitters can do even fancier mixes, not just straight on/off and proportional. With my radio, I have the rudder/motor mixes controlled by a switch, so I can turn them off if I want to.
On the Futaba that I have, you can have several mixes running at the same time with no problems - they all work as you would expect them to.
On my twin-motor Riva I used channel 5 for the second motor/ESC. That way there's no danger of powering one of the motors if the right stick is moved inadvertently.
No need for a BEC Y cable, just pull the red wire from one of the ESC plugs. If you are not comfortable doing this, use a short servo extension with the red wire snipped.
I was talking to a couple of way more experienced modellers at the lake this morning (I've only been into boats for about 6 months - planes and cars before that), and it's surprising how many guys have splashed out on fancy computer radios, and never use the features that they've paid for. I think it might be a good idea to run a "how to set up mixes and stuff" workshop when the weather gets bad. Something that really surprised me was that a couple of guys have fitted mechanical stops on their transmitter sticks to limit sail servo travel, when the Tx can do that for them by adjusting the endpoints.
