As far as iTunes content alone, a network volume with
the iTunes folder on it. Then each computer is set up
to have their iTunes library loaded from it.
The one drawback, for the computers, is that only one computer
can access that library at a time.
The other drawback, with iPhones, is the issue of syncing
everything else if the two computers are not maintained to
sync to each other (contacts, calendars, etc.).
The computer access solution to iTunes itself works quite well.
I have my iTunes library, i.e. complete folder, on a hard drive
connected to my Airport Extreme Base Station. And my MBP,
my work Mac Mini Server, and the Mac Mini I use as an HTPC
all access their iTunes folder to the one I mentioned.
I do however, only sync my iPhone to the Mac Mini Server because
it is the only one that contains all the info I want to be sure to sync.