How to use two ex.HD's to synchronize backup automatically

[lionclaw]

Hi.

I have bought two external hard drives, which I'm gonna use to work with a film project.
I want to use one as the main drive to edit in and import new material, and other one as a backup drive.

So my question is, how I can make the back up drive to automatically back up from the main HD?
Or do I have to keep moving over new data from one to the other manually?
It would be so much effective if the back up drive just automatically back up the other as I add new files and clips to the main HD.

I use MacBook Pro and my HD's are WD passport Ultra.

Thanks!