It's an interesting idea in theory but it just simply would not pan out in actual practice.
I've watched AAA on television a few times and this concept seems to be a popular attraction there. They've had mixed tag titles there since June of 2003 but I think American fans are wired a little differently. In mixed tag matches in AAA, the women often wrestle against the men, usually holding their own. American fans will not take a male wrestler seriously if he wrestles against and gets beaten up by a female wrestler. If WWE adopted such a concept, it'd almost certainly be kept as Diva vs. Diva and Superstar vs. Superstar in the match, only the male wrestlers would be bogged down because most American fans don't care about women's wrestling enough to justify having mixed tag titles. And why should they? Throughout wrestling history, women's wrestling has never been a big draw. Vince knows this, he accepts this and he's not trying to reinvent the proverbial wheel because it'd be a waste of time. In the end, mixed tag titles would ultimately fail in the WWE.
As I said, American fans are wired a little differently. Women's wrestling isn't a huge deal but it can provide entertaining filler for them. They don't want to see men wrestling women nor do they want to see wrestlers restricted by weight classes.
This is a concept that I could see TNA adopting. TNA has had women's tag team titles for a while now that are barely ever used or seen and don't mean anything so this title sort of title would fit right in.