I'm curious about this, too, since I don't have room on my 47" frame, either. But, from observing at triathlons, where you see a lot of double bottle holders, bottles aren't very secure in them, they fly out at the slightest provocation, and the pet name for double bottle holders is Rocket Launcher.
I have seen a sort of cool thing, though, a girl that had a big plastic cup in one side, with all kinds of tubes and stuff in it. Maybe not the most secure, either, though.
Camelbaks are nice, but hot in the summer, so I've taken to carrying extra drinks in a little trunk bag on the rack, (Topeak) but you'd look like a dork with that set-up in a tri...
Nanci