Not as cool but rides where I have access to refills, I use one water bottle.
The second (seat tube cage) is a waterbottle cut in two that I use to carry spare chamois creme, spare batteries, small tripod and a few other things. Slides back together fairly snug so the container doesn't separate with the vibiration.
I've also used it without the top section for easy access to a camera while not wearing a jersey.
wb by
gulpxtreme, on Flickr