This paper presents a study of the impact of horizontal aperture separation in single-sided ventilation flows with two apertures (SS2). The study is based on wind tunnel measurements and dimensional analysis. The results show that the SS2 ventilation flow rate, scaled with incoming wind velocity and aperture area, depends on the incoming wind angle relative to the aperture façade, θ, and on the aperture separation scaled by building width, s′. For most wind angles, the ventilation flow increases as the square-root of s′. This study also identified a novel flow driving mechanism – vortex shedding: when the ventilation openings are on the leeward side of the building and the wind is nearly head-on, the flow is driven by a pumping mechanism due to vortex shedding.