There used to be an app called DMGscissor that would split Mac OS X disk images, but I can't find it anymore, and it was never made into a Universal Binary.🙁
Though the app appears to be gone the functionality is still there. You just have to use the Terminal to do it. I must admit, though - the effort required to make a GUI front-end for the command-line tool in AppleScript Studio wouldn't be much at all, and would be something I'd be interested in making in my spare time.😉