I think your dismissing the p94 a bit prematurely, the p94 has 4 modes which allows you to adjust prop speed on each motor to assist in turning.
Check out the p94's pdf spec's its worth reading
http://www.action-electronics.co.uk/pdfs/P94.pdfNow im no 'ferry expert' but i would have to assume that your ferry's real life counter part will turn one prop faster then the other during turning, or reverse one while the other is in a forward motion, this combined with the bow thruster makes for better manoeuvring.
I have assumed that your craft is a fixed shaft, in which case the p94 will not only do the right job for you, but will also allow better control of the craft in a scale movement, as its functions allow for the prop rotation speeds during turning in conjunction with the rudder(s)
Now i could be wrong about that given you say each prop is independently controlled like the real thing, but you also have the issues with you controlling the model via stick's which is not that easy if each motor is separate, remember your going to need a minimum of 4 channels to control this thing, both sticks forward and back for each motor , one stick controls the rudder and one for the thruster. (left and right motions)
The p94 makes it easy with its mode functions to just use one stick for forwards and reverse, leaving one full stick for your rudder and thruster .
If your craft is using something like rotating korts, then separate function of each motor would be the solution. ( i can't tell from the photo's)
Don't get me wrong i like cheap options when ever possible, but in this case i think your craft will benefit from the extra spent on its electronics.
I can't say any more, the choice is your's.