Unfortunately, there is no way to extract individual songs from a backup file at this time. The library sync functionality also doesn't allow synchronizing only specific songs. Having said that, what you can do is restore backup1, long press a song to start selection mode, tap each song you want to export, tap Share->Export as .msf, then restore backup2, and then use Import->Local File to select that .msf. That will pull in the songs you exported from backup1.
Mike
Mike