Why not a two stage torpedo to solve the problem of the launching submarine having to come close? First stage is a conventional (but maybe larger than usual) t

Why not a two stage torpedo to solve the problem of the launching submarine having to come close? First stage is a conventional (but maybe larger than usual) torpedo that drives--with guidance--from the sub into the 15km range, second stage is the supercavitating torpedo that goes that last 15km or so. Granted, that would require the submarine to have much longer torpedo tubes.