I have two grinders attached to the same VFD. In terms of actually running the motors, you COULD go with different hp motors, but since the VFD has internal trim pots, which should be adjusted to whatever motor is being operated..... you'd likely have whichever motor the trim pots are "tweaked" to running great, and the other motor running off those same settings, which would likely cause poor performance, if the motors were not identical.
I have a page on my website detailing how to wire two motors to one VFD.....
http://www.caffreyknives.net/1_vfd_2_motors.html