Reverse EMF diodes should always be used on inductive DC operated devices, especially if the operating device is solid state.
I always keep a quantity of 1N4005 or 1N4007's for this purpose.
All AC inductive devices should have a R/C snubber across the coil, I find this is better than a MOV, which is often used also.
The main problem with using two different motors on the same VFD is that you cannot use the self tuning feature, or tune to one particular motor , due to the different characteristics between them.
You could look into a motor contactor reverser and suitable circuit, this can be wired for two motors and has electrical interlock as well as a mechanical interlock, preventing both from picking up.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.