Backing up iPhoto external HD

II've got to admit, I'm baffled and my macbook hd is now full!

Up until now I have only used time machine to backup my macbook. Now I've got kids my photos have become so much more valuable and I have purchased two external hard drives with a view to using them for backing up. My confidence has been rattled further as the last drive I bought failed!

Originally I thought I would set both up in two different formats incase one day I don't have a mac or my wife would like to use the photos on her PC.

Ive just formatted one drive to mac extended journalled but just thought what happens when I take more photos?

What does anyone recommend?

Can I set one drive to be my main iphoto library freeing up hd space on my macbook, keep the other for making back ups every now and then and still back everything up through time machine?

OR is there a better way to do this?

Thanks in advance for your suggestions