This might sound crazy, but would it be possible to have two hard drives, each dedicated to one operating system, in the same machine. Have both hard drives set up as master. Then take the power supply cables from both and rig up a on-off-on switch so that you can manually choose which hard drive is powered before bootup.

OK now tell me I'm crazy.

Everybody that I know, that has had a dual boot setup on their machine, eventually went back to just one OS.

What about keeping both machines running the way they are and using a network storage device for backup for both machines? Not sure on the compatability issues though.