4TB mybook to backup 2 PC's?

Just bought a 4 TB Mybook and want to backup 2 different computers, one running Win 8.1 , the other running Win 7. How should I set up the Mybook drive ? Will I need to partition into 2 2TB partitions or can I just run a backup off of each PC ?
If I have a backup from each, will the drive know which backup set to write to when it’s plugged into the pc for the next backup?