Surabar was a human - but you're right, given that he seems to have been a very powerful magic user meens he could have changed form, altered the course of a pregnancy, or even just had a bunch of different lovers, some of whom weren't humans - I believe the background trait suggests that Surabar is a remote relative, so there's no reason why a PC would have to be either human or a half-breed of some kind to take it.
But a HUMAN and a DWARF? Unless your game has half-dwarves, it doesn't make much sense for both of them to have it. I'd limit it to 1 player or the other unfortunately.
Or you could change the feat. Instead of it being "a descendant" of Surabar, you could invent some order of magical knights that fought at Surabar's side. The feat goes to a descendent of any of those knights. The knights can then be any race. I'd find some way to insert some of that information into the adventures.