Nobody?
I remember an iTunes hack (workaround) where you could make an alias of the library file so multiple users would share it. What I am thinking of is significantly more complex. I imagine you could do something like synchronizing files, but you would not want to use either machine without plugging in the hard drive, which is not really very convenient...
Surely someone must have done this, or is wanting to other than me!
EDIT:
Well, you know, I'm probably just thinking about one of the other hundred ways my computing life could be easier and more efficient. Perhaps I should learn some applescript. I'm always thinking of simple browser addons as well, like a [right click>look up in Wikipedia in new tab], obvious stuff like that...