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.