This paper investigates the seismic performance of two adjacent buildings connected by viscous dampers. Three types of damper placement are discussed, including installing dampers within a single building, connecting two buildings at the same floor level, and connecting two buildings at the inter-story level. Analytical models are established to consider various dynamic properties of the adjacent buildings, and the theoretical solutions are obtained, including the transmissibility curves, additional modal damping, and input energy under the seismic design spectrum. Time history analyses of an engineering project are performed with different damper placements. Different numerical models are compared for frequently and rarely occurred earthquakes. The seismic mitigation effect is discussed with regard to the story drift reduction rate and dynamic energy. Theoretical and numerical results demonstrate that the connecting dampers provide added modal damping while causing the coupled response. As a result, it is less efficient than traditional ways of placement within a building. Furthermore, the connecting dampers significantly increase the reaction of the floors without installed dampers. When designing dampers to connect the adjacent buildings, careful engineering calculations should be made.