Cross pollination of these two species has been done, however the process is patented. As far as I know only Sakata has used it. As I understand the pollen from one species is chemically treated to make it acceptable to the other species. I doubt that it will work in a natural setting, but you have nothing to lose by trying.

Well, it seems to have worked; the fruit is already the size of a small apple! It remains to be seen if it will actually mature it though. And of course if it will be worth it - I keep reading about the Futsu's "hazelnut" taste but people who've tried them say they couldn't taste anything like that. I also see conflicting reports on Rouge Vif d'Etampes - some say deliciously sweet smooth flesh, others say fibrous and not very sweet. Of the ones I'm growing, Marina di Chioggia is the only one I've read consistenly good things about!