Possible, but not advisable.
You would be using the same drive for backup & normal (live) uses at the same time. That pretty much negates the whole purpose of backup, at least as far as that single drive would be concerned.
Better to devote 1 drive to backup (TM or otherwise) and a 2nd drive for online (live) usage.
One more thing ... you don't really have effective backup until you have at least 2 separate backups on 2 separate drives, neither of which is connected to your Mac except for backup & restore operations. Three is even better.
ds94901 said they would use two drives, one for iTunes and one for backup.
That does not seem like a bad solution to me.
iTunes requires you to first specify which is your backup drive. Once you do that, it will try to back up everything else. If there are things you do NOT want backed up, you click ( Options ) and it will let you add drives, directories, or files to the BlackList.
You are right that we should have multiple backups, but one is SO much more than none!