Why not use a backup application that does incremental backups, i.e. only copies new or modified files to the library on the EHD. This will give you only one backup copy, the most current. It would be advisable to occasionally open any backup copy to make sure it works as intended and does contain what you expect it to.
I use Synk Pro and have it setup to automatically copy only new or modified files immediately upon closing iPhoto. There are other apps that can do incremental backups. You can search for them a MacUpdate.com.
I am not a fan of Time Machine. I back up iPhoto to an external drive manually. The only way I've been able to figure out how to do it is to save a copy of the current iPhoto Library to the external drive. However, then I end up with multiple copies of the iPhoto library on the external drive as after I do the backup I delete some of the older events off the Mac hard drive for memory purposes. What I want to be able to do is to slide old events into one iPhoto folder on the backup drive so that I keep one complete copy of all of the pics there. I don't think it's possible to do this. Yes or no? If not, what would you suggest as the best backup method outside of Time Machine?