We present a simulation study on the formation of particle arches supported by flying buttresses that arrest the flow of monodisperse spherical particles through two adjacent slots (rectangular outlets) in a three-dimensional (3D) anisometric flat-bottomed silo discharging under gravity. Most previous experimental and simulation studies on the jamming of a silo have focused on a single outlet, and have shown that themean number of particles exiting the silo before it jams depends on the ratio of outlet size to particle diameter (R). In this paper, we look at the nature of jamming if there are two adjacent outlets and discover some new physical insights on how sand arches are mechanically supported and how they interact with each other. For a fixed slot width Do and particle diameter d, the distance separating the two openings Dw is varied. The distribution of N particles exiting the silo before both slots are jammed is obtained for each Dw. From these distributions, we show that as Dw becomes less than approximately 3d, the particle arches on adjacent outlets start affecting each other, and mutually stable arches become harder to form. We show that in cases, where for a small orifice ratio R = Do/d a single outlet would have easily jammed, two adjacent outlets do not jam.We propose that this is due to the importance of stable particles resting on the base of the silo adjacent to the arches, which redistribute the load much like a flying buttress does in Gothic arches.