How does Time Machine handle an external drive shared between two Macs?

Question

I have a new MacBook Air (13" with 128Gb SSD) and my wife has an old MacBook (120Gb HDD) and we're planning to both use the same 1Tb external HDD to store our iTunes and iPhoto libraries and the same 2Tb external HDD for Time Machine backups (obviously only one of us can use the external HDDs at the same time).

If each of us sets the the 1Tb (music and photos) drive to be backed up by Time Machine will Time Machine back it up independently for each user, even though the data is the same? If so (which I suspect is the case) then I guess it would make sense for only one of us to configure the 1Tb ext HDD to be backed up by Time Machine, with the risk that any changes the other person makes won't be backed up until the one doing the backups connects?