No idea but it sounds like they're having fun trying!
I suppose they are different species even if closely related (it's 20 blah years since I did biology and haven't really though about it in the interim) but horses and zebras can mate and horses and donkeys, because the courtship behaviour is similar enough for each species to understand the other, but the offspring are infertile. But that doesn't help at all really - I think what I'm suggesting is that they should be able to successfully mate but the offspring may be unviable.
Sorry!