Well, sometimes you can get more torque out of the system, or you can also get a more direct coupling between the surfaces and the servo. The latter happens because most single servo setups use a torque rod or some sort of bellcrank mechanism to actuate the ailerons, but most dual servo setups are mounted outboard on the wings and have a simple pushrod out to the torque arms. I prefer them, but I also have a computer radio and don't need to hook up a Y-cable.
Curtis