I was never a fan of the proposed refit, I have seen the drawings but they ever say what the technical reasons for having a secondary hull would be? More space, more power etc?
It's explained in the centerfold of the Ships of the Line 2011 Calendar
. It was said to be primarily about making room for a larger, more powerful warp reactor. It also let them add a larger deflector dish to supplement the "inadequate" one on the front of the saucer.