Backup behavior on multiple computers

I have a laptop on Windows 7 and an older desktop on Windows XP.  I installed the WD backup software that comes loaded on the Passport Essential SE hard drive on both machines and individually ran the backup.    The software creates a unique “user” name for each PC.  The files are segregated (albeit a bit hard to find, but definately segregated).  Easy for backing up multiple computers on the same hard drive.