[meatmanek]

It looks like from what you're telling me, one motor is faster and lower torque, and one motor is slower and higher torque. Gear the slower one up, gear the faster one down, so that they both have roughly the same torque and the same free speed.
As for the backdriving problem, perhaps you could gear the window motor up so that it's slightly faster than the globe motor. This would make it so that the globe motor won't try to overdrive the window motor. Another solution could be found in software - make sure you don't power the globe motor faster than the window motor. This might require more than just a linear scaling function.

They're right, though, you're better off using 2 identical or similar motors.