Depends if you want it automatic or manual, but the easiest way I would think is to have a power relay for each with each having a N.C. aux contact.
The aux contact would be wired in the circuit of the opposite contactor or relay coil.
IOW, an electrical interlock.
The other similar method is to pick up a reversing contactor off ebay, these have mechanical interlock as well as electrical.
Using it this way it would obviously supply one of two motors rather than reverse one.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.