Embodiments of the disclosure relate to a method of controlling the flow of fluid, such as air, between a stack of glass sheets during a co-sagging process. In embodiments, this involves a particular method and certain mechanical means of applying force at or near the edges and/or corners of a stack of glass sheets during a co-sagging process. In other embodiments, this involves creating low pressure regions at or near the edges and/or corners during the co-sagging process. In particular, controlling the flow of fluid between glass sheets is particularly suitable for preventing shape mismatch between two glass sheets having different thicknesses and/or compositions.