Tatelyle, the wiki page for the project reveals some reasons for the design being as it is. They're taking a lot of hardware from two 747s, especially the landing gear, so that will have a lot of bearing on the fuselage design. Linking the stabilisers looks like a good idea at first, but the effect of the payload on the flow over a joined stab might be interesting, maybe more so as it departs. If the drop goes a bit awry it at least gives a chance of avoiding it hitting something too.