If you want to put your second water bottle behind you (as in the photos) and have a seat bag on your bike, Minoura makes a nice water bottle cage holder that mounts on either the handlebar or seat post. The nice thing about this mount is that it can be set to any angle so that it can be mounted on a seat post and not interfere with with your seat bag. I have one and mount it on my road bike on long rides as a third water bottle for my recovery drink.