This is a good question. I'm thinking of chipping in and suprising the wife with one when I finally take the dive for one myself. She doesn't even know she wants it yet, but I know her, she just doesn't understand how baddass this phone really is. She will see the light!
I sync an iPhone, two Nanos, a Shuffle, a ROKR, and a SLVR on the same computer. They all sync perfectly through iTunes. OF course only the iPHone has to sync contacts and calendar. I think because of that, there may be a difference in how iTunes treats different iPhone devices. How would iTunes distinguish which calendar and contacts to sync to the second iPhone? It's a good question that you could probably find an answer to on the Apple.com support page.
My wife and i share a computer and each have an iPhone but we use different sign on names on the comp. we have a vista machine so we each have our own iTunes library. As far as syncing ipods and iPhone we both also have a ipod and sync to the same iTunes we sync our phones to.
I don't think you can use the same iTunes for two iPhones though but its definitely not a problem using the same cmputer