They probably would...if not naturally, then with help. It is likely that the flowering time is sufficiently offset that creating a hybrid will require storing pollen.

So...that would mean you couldn't plant a swamp tupelo in the wettest part of your yard and a regular one in the driest and expect to get berries?

I'm speculating on differing flowering times being the reason there is a lack of documented hybrids. It *could* be genetic incompatibility but interspecific hybridization is pretty common in plants.

Even if they don't flower together, you might get berries if they are self-fertile, but I don't know if they are. They wouldn't be hybrids in that case however.

Well my goal (to the extent I have one) is to stick a female hybrid in a zone 6 swampy area, and then stick a fastigate cultivar of Nyssa sylvatica close to the house where it is drier...

I have both Ns and Nb on my place. The Nb grows in the wet areas along the creek and the Ns grows on drier upland areas. I've never paid attention to bloom time, but they both produce fruit regularly. Beside the blossom, the only difference I can see between the two is Nb has more horizontal branching and Ns is more oval-upright. They both leaf out the same time and color deep red the same time in fall. I don't think I've seen any that I thought were intermediate, but I really haven't looked.