The Tamiya drive is a loose and incredibly noisy affair. But if you can find the Yamato version, it has 4 outs.

I'd go with separate servo drives myself, as I did on my landing craft:
http://www.geocities.com/patsmodels/lcm3/
Also look into the NWSL couplers.
Note- I have a single 6v 380 driving the stock 4 props on a 3 lb Yamato/Musashi, and it goes well over scale speed. You don't need a lot of power here. http://www.geocities.com/patsmodels/musashi/

If the servo motors are just a tad too small, you can use any small 6v motor (280 size) wired to the servo board. The bigger motor won't draw much current driving the small model props, unless stalled. Then a new servo board is only $9.99!
More on servo hacking at: http://www.geocities.com/patsmodels/servo/

Pat M