All members of the cowpea family can cross under some conditions. But the 'some conditions' is rare to very rare. Cowpeas have a large legume type bloom that encloses the pistil which is pollinated just before the bloom opens. However, bumblebees and some other pollinators can rip the bloom open prematurely and once in a while will cause crosspollination.

You don't have to do much to prevent crossing. Plant them at least 25 feet apart in rows and have taller intervening plants such as corn between. This won't give 100% pure seed at breeder standards, but it will be close enough for home growing.

From my perspective, a few hybrids between the yardlong beans might be desirable. You could come up with something unique and better producing than the parents.

DarJones

Thanks for the info DarJones, since I have the seed for both I'll try growing them, nothing ventured, nothing gained. Now all I need is a summer like we had last year.

Annette