A model of single-room displacement ventilation is extended to a space consisting of two chambers of equal height connected by two openings. Individually, both chambers have displacement ventilation in this geometrical arrangement, but the space itself is not connected to the outside. Thus we are considering ventilation of two chambers in the interior of a building, such as an office connected to an internal atrium. Theoretical analysis and experimental results are presented in this paper. The experiments use salt solutions to simulate thermal forcing in buildings and the theoretical analysis is based on plume theory. The two chambers have a time-dependent interaction resulting from changing stratification in the two chambers. We concentrate here on a small chamber with an internal heat source connected to a large unheated chamber, and show that the time variation is determined by the size of the larger chamber. We discuss the implications of these results for building ventilation design and control.