You can't sync any iOS device to more than one computer.
In order to have a backup of your backup, you should probably backup your computer instead.
If you just want to have an extra copy of your iOS device backup here is its location: ~user/Library/Application Support/MobileSync/Backup
Keep in mind that this backup doesn't include any media files which are kept in iTunes. That's why I was mentioning that you should consider to have a full backup of your computer instead like using TimeMachine.