My head just exploded.
I can't tell what you want to do.
If you merely want to backup your itunes library to the external disk, then time machine is doing what you desire.
If you want to move your iTunes library off to your external drive, you can either reconsolidate the library to a seperate partition on the external, or you can create a symlink that stores just the music files on your external drive while leaving your library attatched. This way, iTunes won't complain everytime you start up the app without your drive connection (in the case of iPhones and such).
If you want your entire library stored on the external and then duplicated back to your internal drive...
//head explodes again
edit:it sounds like you just want time machine to backup your files while leaving the master copies on your internal HD so you can listen to them whenever you like.
Time machine already does this. Basically, you can ignore it's function-that is to say that whatever Time machine is backing up will have no influence on how you already utilize your box, so just having time machine enabled won't make iTunes ask for the drive. iTunes won't even know it exists.