Use of a second stage necklace is perfectly acceptable for back mount diving and is part of the configuration for tech divers. Even if both second stages are connected to the same tank using a necklace to hold the back up keeps it in a well known and convenient location. But the also assumes that if you need to provide air to a buddy that you will then provide your primary and then switch to you back up. This is different from what is taught in open water classes where you should prevent/block the diver in need from taking your primary and offer them your back up. In short it is a matter of choice for the diver.