These old building were nice in that they fitted together with a nice tongue and groove on the back. But to go the extra mile, I've been using putty to fill the gaps between two glued-together buildings.
The filler I'm using is Revell's "plasto". Unlike green stuff, which has the consistency of chewing gum, this stuff is more like car body filler, or cement, and it's applied in the same way: squeeze a dollop out of the tube onto the relevant area, then use a spreader to shape it, and then file smooth once it's dried.
The finished surface of the affected areas will be visibly different from the rest of the building, so I will probably end up applying a thin coat of the filler to the roof and sides of the buildings, just to keep it consistent.