I had that set up on my second road bike. Compact frame for shortie like me left room for only one bottle cage mount. Riding in NC summers - one bottle of fluid wasn't going to cut it.

I bought a metal extension thing from Minoura that attached to the base of my saddle (not to my seat post since I hardly had any showing!) and had two standard sized cage mounts on it. It worked great!

The only problem I had was that on really rough spots (you know, those spots you shouldn't ever hit on a road bike when doing 30 mph? yeah, those ), the bottles might bounce out.

Ha! Found a link:
http://www.amazon.com/MINOURA-WATER-.../dp/B001N5EJPI

I also have a photo of it on my bike while I'm riding it at home. I can post it later if you want to see how it looks.