I understand the concept of multiple backups in sync, but in my mind that is not all that practical. I do what I consider the next best thing, which I use Time Machine with two externals, and I alternate which drive I back up to. That way I have 2 backups of everything up to 2 backups ago, and 1 backup of everything through to the last backup. That's very close to 2 in-sync backups; odds are I won't lose both, and if I lose the one that includes the latest backup, I am at least covered for everything before that.
But Time Machine is not perfect; it has no real granularity. I use a free utility called Time Machine Manager (?) widely available on the web that adds that granularity. What you do is turn TM off, and use the TMM utility to adjust things. Otherwise, Time Machine wants to make so many backups every other minute that your HDDs fill up pretty fast.
You might look at SuperDuper, especially if you want bootable backups.
But while I am urging you to have multiple backups of everything, what you really asked about is the iTunes library and media. Of course TM or SuperDuper would make that a moot point should you want to back everything up, but assuming you want to just keep the iTunes stuff backed up, I would try to see if iCloud would be a solution (not sure how it handles multiple external drives, even if connected to a single computer possibly only one at a time). Of course if you had more than one computer with your iTunes library iCloud would work just fine keeping it all in sync.
You might also consider a NAS, such as a 2 TB for about $240 from iOmega. Put your iTunes library and media on there, and point iTunes on each of your computers to that. Since it is one library, it is automatically in sync. THat works unless you need to access iTunes away from home on a laptop (which if you have iCloud and internet connectivity is not really an issue). Back that up with another HDD using TM or SD, and you're golden. Then put that on iCloud for an offsite backup, and all will be in sync.
In the past I have kept multiple libraries on multiple Macs in sync using SyncOTunes, but it is now abandonware, and it does not move the metadata, just the media, so stars ratings etc will not track from computer to computer. Its still out there for free, however.
I tried to get cute and move just the library files and media from one computer to the next but it ended up in a big mess. It also skunked my Alarm Clock program which wakes me up every day with a iTunes playlist that starts at a low volume and steadily increases over a 15-minute span. I ended up having to delete iTunes and all the media completely, reinstall iTunes, Alarm Clock, and start from scratch. I do NOT recommend that; it took me an entire day to straighten everything out.
