Masonry buildings form building aggregates around the world in historical centers, which developed as the layout of the city or village densified. In these aggregates, adjacent buildings can share structural walls, connected at the interfaces by interlocking stones or by a layer of mortar. Observations after for example the recent earthquakes in Croatia often showed signs of separation and pounding at interfaces due to out- of-phase behaviour of units. The analysis of such building aggregates is complicated by the lack of guidelines, as the scarce experimental data hampered the advances. Therefore, the objective of the project was to generate such data by testing an aggregate of two buildings under bidirectional dynamic excitation. The test unit was constructed at half- scale, with a two-storey building and a one-storey building. The buildings shared one common wall with the façade walls connected by a layer of mortar. The floors were at different heights and had different beam orientations to facilitate the out-of-phase response. The shake table test was conducted at the LNEC seismic testing facility in Lisbon, Portugal. Interaction between the units was observed both in the longitudinal and transversal directions. In the transversal direction, frictional forces played a role at the interface between the units. In the longitudinal direction, signs of pounding at the interface were observed, together with the formation of a soft storey mechanism at the upper storey of the higher unit, starting from the point of interaction.