Yongbo zuner.net
There is no way you can share the same computer and two different music collections.Why not just have one collection and each sync only what you listen to.You can even setup your own playlist.One is linked and the other is a guest.
You can have both Zunes linked to one computer. That's what my brother and I do. You have to keep all the music that both of you want in the collection but you can pick and choose what gets synced to what Zune. And if something gets synced to one Zune that you don't want you can delete it without deleting it from the other one.
You just have to keep everything in the same collection.
Zune 1up