Hi Grant, you can indeed use a Y lead to plug both ESC's into the receiver and so use a two channel set. the radio battery idea is a none starter :-\
BEC indicates that the receiver can accept a power input from a speed controller of the right type ( Electonise are not) through a socket other than the one marked for battery. what it does is to fire the power + and - from an output section of the speed controller to the refceiver, and the third "signal wire" brings the control signal back to the ESC.
Electronise are cracking speed controllers, but you will have to carry on using a battery for the receiver. Incidentaly, one of our members was in the local £1 shop at the weekend and they were selling packs of 4 AA size Nicads, 800mAh for - you guessed it, £1
One thing you could try adding if you want the boat to turn a little tighter while still only using two channels is to connect a pair of microswitches to the rudder servo and use them to switch off the inside motor on turns. My old mum ( 77 and still building and sailing
) has these in two of her twin screw models - Waveney class Lifeboat and "Lochmor" Scottish ferry - and they make a big difference.
If you want it making clearer, drop me a PM and I will do a wiring drawing for you.
Regards
Aston