I think this can be done if both sites are on the same server or both can access the same database.
Without changing any files (which are encoded) they would share the same settings, clients, products, everything you can set in WHMCS would be shared.
I think the only way you could theme both of the sites would to use the same skin name, but keep different files in the directory, since it would share the skin settings.
It would required more than one licenses and I think thats where the only problem would be.