I'd pronounce it with a brief middle syllable: BEH-tә-nee. But when you say it quickly the middle syllable kind of gets eaten up, so it almost sounds like 2.
I think it's kind of nice, actually. I never would have thought of it, and I don't think I'd ever use it, but it's nice. I think it's a little too matchy with Bryony to use them for twin names, but I like them separately.