Welcome to the community, bbunch06!
Yes, you should be able to backup both of your systems on the WD My Book external. However, the WD SmartWare software is strictly for data backup, meaning it wouldn't include the OS files. Here's the user manual for the SmartWare backup software:
http://products.wdc.com/support/kb.ashx?id=spgXvR
But in case you want to backup (create a system image including the OS backup), I'd recommend you to take a look at Acronis True Image WD Edition here:
http://products.wdc.com/support/kb.ashx?id=sau34K
Among its other features, it can also create Drive Image Backup which would let you create backup images of your operating systems' HDDs that can be stored as a file on the WD My Book.
Hope this helps. Let me know if you have more questions!
SuperSoph_WD