Hi Myrtle and welcome to the vine! Theres a lovely bunch of likeminded grapes on here, some of whom have loads of experience and share it willingly, so usually someone will come up with an answer for you!
I'm afraid as a fellow newbie I cant add much to whats already been said, I think the incompatability comes from the nutrients that both plants need, if they both are seeking the same food, then they will be in competition, so wont do as well as they would separately, I'm sure thats what my Old Dad told me many years ago! I may well be wrong tho, so dont take it as gospel!