Efficiency is the key to everything brushless, a few percentage points efficiency can mean the difference between a setup that runs cool and one that runs hot. You're almost always going to be better off with a bigger motor than two smaller ones. Do you remember the traxxas E-maxx twin VXL concept? Not only was this never released to the public (for it's brushless MTs, traxxas ended up going with a OEM castle creations system), but if you look carefully the ESCs were joined together with some kind of communication cable between the two. Presumably to syncronize the otherwise sensorless brushless ESCs and keep them from fighting each other, which would contribute to inefficiency.

With brushed this was never much of a problem since the switching of the motors depends completely on the shaft position. But with brushless, if you have each ESC switching independently, in the best case, you've lost some efficiency, in the worst case you might strip a spur gear or something similar (if you have two motors that tend to fight each other really hard).

There are some applications where multiple motors might make sense, but we haven't seen many of those enter into RC as of yet. Most of these applications call for motors that are not on the same transmission system, but instead each motor drives one or two wheels and then by varying the power or speed of one (or more) motors intelligently, the user has an advantage in control. For instance, a dual MOA (Motor-on-axle) crawler where the motors are run or stopped selectively for a 'dig'. Or even a motor-to-wheel direct drive, which could in theory steer without the need to yaw the wheels in the horizontal plane, but merely by increasing or decreasing the speed of one's side wheels.

And don't think I know everything, there have been times when I've been corrected by others here. But I think most everyone has in the past agreed with the dual vs. single motor concept. It's like how you don't see dual-engine cars in existance... if more power is the end goal, it's easier, cheaper, and poses less problems to just increase the size of the engine.

The above post mentioned an AC motor, and this is a slightly different case since, other than at spin-up and spin-down, the phase and frequency of the AC solely control the position of the rotor. So, even if completely independent (not on the same shaft), two AC motors run from the same AC source (or inverter) should eventually 'sync up' once they reach their top speed (a factor of the AC frequency, provided there is enough current for a given load). Though I could be wrong, AC motors aren't my thing, I've been wrong about stuff before.