Mount additional water bottle on bike frame with only one water bottle mount
Get yourself two hose clamps that'll fit around the tube and some rubber strips to put under then so they don't scratch the paint up too badly. They'll hold a bottle cage on just fine. 

If you can see under the dirt and grime, that's exactly how the bottle cage in this picture is mounted: 



Edit: Here's another picture of a bottle cage mounted with hose clamps (not mine).