Yes. Thanks for the tips.
I got my x399m board and dual cards. So far so good. I choose shorter cards.
There is still a small gap between two cards anyway. I think that still help cooling inside the case.
I am curious about how you insert 2 GPU cards on X399M. Is it possible to use the 1st and 2nd PCI-E slots together? Do you have any pics for that? Thanks.