In what way does your solution share the backend? Does it use different databases and cache/public/upload/hooks-folders, or do you have it all setup to run on the same db/folders?
1 IPB install. Many domains pointing to it. Different output based on domain.
TO clarify, I don't use this live, thus I do not know the challenges re: cookies, etc. It's something I made to 'see if I could'.