Pick up any standard three phase reversing contactor/switch. Remove the load side jumper wires and viola you have an set of interlocked of contacts. You will need to add individual overload relays because each motor has a different FLA. Then you need to add interlocking to make sure the VFD is "off" before switching any motor "on" (VFDs do not like it when they are up-to-speed and a motor is "slammed" on).
After installation it may be less expensive to simply purchase a second VFD.
