Also, you could probably use a motor controller that has feedback for both of the currents of each of the motors and close-loop control the torque in each motor and also control the RPM. This way, you wouldn't accidentally drive one motor more and then think its an increased load, when you're actually driving the other motor as a generator.
Actually, I can point you towards a project with 2 motors coupled together at the shaft that I did where I modeled a common shaft system in MATLAB, but they weren't BLDC. Do you have a messenger service? I'd rather not give the project information out here since it has my name all over it and I'd like to keep my identity private. I could also ask my BLDC questions more easily.