Ben,
Two seperately controled motors means two ESCs and two channels for throttles. Plus the other channel for the rudder and you have three channels required. It won't be the same as having dual throttles on the same 'quadrant' but you can do the controlling by using both 'sticks' of a 4 channel radio, for instance. Up/down both sticks for throttle and left/right on one stick for rudder. There are a couple of 'dual throttle' radios but you're talking about a sort of 'high end' radio sy$tem. Expense is relative, so that's up to you. The 'cheap-n-dirty' method is to just parallel the two motors on one ESC/throttle. 30 knots, depending on the scale of the boat isn't gonna be all that fast, so shouldn't be that difficult.
Have fun.
- 'Doc