Most modern VFD inverters are actually sensorless vector drives, not just simple three phase voltage sources. They try to drive a motor at constant speed, compensating the slip and enable the motor to produce maximal torque over the full speed range. They may act in an unwanted way, if the parallel connected motors are loaded with very different torque. Running the motors "onto the same load / output shaft" should work fine.
In case of unbalanced load, you should consult the inverter user manual. It may offer different control algorithms respectively reducing the drive to a thumb voltage source.