Another option, if your radio is capable of it, is to set up a mix in the radio. I use this, and my setup uses an individual ESC for each shaft. The Port ESC/Shaft is controlled by the left hand stick on the radio, in my case, a ratcheting throttle stick. The Stbd ESC/Shaft is controll by the elevator, or R/H stick. The rudder is independent and controlled by the alieron channel. I'd give channel numbers, but it would be for Futaba and might not make sense on other radios. The mix works as follows, when the mix is turned on, the elevator channel is slaved to the throttle, giving me single stick control over both motors. Great for just motoring around. When the mix is off, the throttle controls the Port shaft, and the elevator controls the Stbd. So far this has worked out very well for me, I find it much easier than trying to manipulate both sticks at the same time.
Having said that, if you cruise Ebay, on occasion you can find an ACE Nautical commander radio, it was set up from the factory with twin throttle controls, duplicating a full size boat. The problem is that they're very old technology and need to be updated before they're really usefull.