I have used multiple betas once or twice, but only when I was hearing something from the primary beta that felt wrong to me, or when I knew I was in a dicey situation with a story and wanted another opinion. JustTripn was a wonderful beta, but sometimes she and I just fundamentally disagreed about something.
And do you really think betas influence a story's style all that much? It's not like we actually rewrite -- we just copyedit and give feedback. I wouldn't say you can detect much Alelou at work in Putaro's work. It's very much his own, and he's good at rejecting my suggestions when they don't fit his vision.

I do think there's some advantage to working with someone from the other gender, though. If nothing else, it forces you to think through some of the choices you're making for characters in ways you might not otherwise. I know when I'm writing there's an internal chorus of male reviewers from the past that I can listen to, and predict what will outrage them (and God knows I've outraged them). It may have helped me make Trip a little more acceptably "manly" in later fics.