Possible to Connect One Driven System with Two?

It is possible if one motor stator is free to rotate with the speed of the other motor. The supply for the rotating stator being done via slip-rings.

Unfortunately stators (as their name says) are not designed to rotate but to stay.

You can also use a planetary gear to sum the rotations but you will also need some brakes to be sure that you can use each motor separately.

A way to do it is to drive a common rotating shaft:

On the 1500 rpm motor you put a pulley, sprocket, gear or contact wheel with a diameter, 1,33 times the diameter of the same on the common shaft, and on the 500 rpm motor the same with a diameter of 4 times the one on the common shaft.

For the gear, one motor needs to run in the opposite way. With a contact wheel, you can do it with one wheel on the common shaft.

For sprocket or pulley you better work it double, however in theory it can also be done with one, if you play with the positions. This example is for parallel running shafts: 1.33/1/4

Just adding rpm's doesn't work.

VFD

Why ?

only if you drive the 1500 rpm motor ie spin the whole assy at 500 rpm then run the 1500 rpm motor

but i cant see how you would add the two speeds toghter any other way to reach 2000 rpm.

but why do you want to use two different motors ?

why not one via a gear box