Just keep in mind RAID is NOT backup. Even if you place the library on a RAID array you will still need to back it up.
As for using the library on both systems placing it on an external drive is the easiest solution. Aperture doesn't lend itself very well to being shared between systems, there i no good way to keep separate libraries in sync. So having one library on an external drive that is moved between systems is best.
For sharing the library between Mac's then yes putting the library on an external dive and moving it between systems is the best way to go.
For backup, RAID isn't needed (it does provide benefits but backup isn;t one of them) you can use Time Machine. Normally external drives are excluded but you can go into TM's settings and add the drive. You could have both systems backing up the drive by TM when the drive is plugged into that machine. Just be aware if the drive was not plugged into one of the systems for an extended period and you did work on the library during that time, when you do plug the drive in the backup could take a while.
Adding a clone of the external drive with the library on it would be a good idea. While TM is good it isn't an archival backup so for images especially, I like to add a clone.