You need a large space open to attach the rocket to, and it has to be at the center of lift. You can't have one fuselage and put the rocket to the side, because

You need a large space open to attach the rocket to, and it has to be at the center of lift. You can't have one fuselage and put the rocket to the side, because then the balance would change drastically when you add the rocket, and you can't put the rocket under/in the fuselage because it's very large.