Since the White Knight Two doesn't transport passengers or cargo, why does it even have a fuselage? Wouldn't a purely wing shaped aircraft be better? Is it simply more convenient to use an existing and tested aircraft design, even if it was created for a different purpose?
One fuselage is an exact replica of that of SpaceShipTwo (to allow tourist training), and the other will carry 'cut-rate' day-trippers into the stratosphere.
Why even have one? Well, there is a pilot of White Knight. Given that you want to have Space Ship two be in the middle, White Knight needs to have two fuselages, to maintain stability with and without the SpaceShip Two.
Bottom line, having two fuselages makes sense for stability, and if you have them anyways, why not make them useful?