Would this be a valid way to turn on either one or both motors using two relays?

 

As long as the vdrop and current load on the diode is an acceptable loss, Alternatively the same idea can be applied to the relay coils instead of the motor drive, eg signal applied before the diode powers 1st and 2nd relays coil and signal applied after diode only powers 2nd coil.

 

Thats a great idea.
Thank you!