I had this idea running around in my head about connecting my two 55g's with a 12" cylinder, acrylic tunnel to basically make it one tank. I did a little searching and couldn't find anybody that had done it besides this guy...
http://www.ohgizmo.com/2008/01/18/fishtank-friday-fish-overpass/
http://www.ohgizmo.com/2008/01/18/fishtank-friday-fish-overpass/