There's no way there will be simultaneous sharing. Just logging in to the network, it would check the s/n against its database, and when the 2nd user tries with the same s/n, it will say, "Someone else is already playing with this copy. Returning to Dashboard."
It may even let you know which GamerTag is using the copy.
Even way back in 1990, Adobe had this running in Photoshop and Illustrator, to prevent users having the same s/n copy running on the LAN. Microsoft's just going to use the same idea on the scale of the XBL network.