I've not actually modeled multiple buildings, but on a recent project I modeled a single high-rise building that was part of a street canyon (maybe this is what you mean). I used the adiabatic boundary condition at adjacencies with neighboring buildings. This is a reasonable approximation if:

1. The building are actually shoulder-to-shoulder (no wind or solar radiation can get between them)
2. The indoor air temperature in the two buildings are the same and/or the effective R-value between the two buildings is high.