2 motors with different speeds in one Gearbox

[ToddF]

When using mismatched motors, at some point in the speed range, one or the other motor will be backdriven by the other. As a mechanical guy, I have to ask the question to the electrical guys: What is the effect on typical motor drivers (such as the Jags) of having a motor being back driven and acting like a generator when the driver is trying to turn it the other way?

I suspect the short term effect isn't catastrophic, as under-geared robots get pushed backwards quite often. But, I suspect that having this happen continuously, as might happen in a mismatched motor system, might cause something to overheat and fail.

Anyone know?