Trying to join two Western Digital My Book Essential drives together.

[Musashi-1950]

I have a 2 x 2tb Western Digital My Book Essential external drives.
I would likw to link them to make a 4tb backup system.
Can this be achieved ?

 

[kenrivers]

http://lifehacker.com/5986883/how-to-combine-multiple-hard-drives-into-one-volume-for-cheap-high+capacity-storage
The above link has some things you might can try. I'm not sure if it applied to external drives but it may be worth a read.