Thanks Ether for your thorough post (as always!)
I realize this is not quite the correct thread for different motors with same controllers on one gearbox. But to close on that one. Based on your posted Torque/RPM charts, in most properly sized applications one is not likely to run anywhere near the free running speed of the slower motor.
On the correct thread, I do think one needs to be careful when mixing motor controllers. The easiest way would be to check the current into each motor with a DC Amp Clampmeter and make sure they are reasonably close. The power dissipation in the motor is roughly I^2R*, so any difference in I(A) will cause an exponential difference in Power per motor. To fix it you can use a LUP (as mentioned before) but given all the hassle - I would just avoid it - its bad practice anyway.
*R is a fusion of restive R and back-emf, but since they should be nearly the same for same motors its a fair assumption (I see a sledge hammer coming down on me from Ether on this one?
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