Two-layer coating occurs in many products. Ideally, the liquids are deposited onto the substrate simultaneously. In the case of two-layer slot coating, the interlayer between the coating liquids is subjected to enormous shearing. This may lead to flow instabilities that ruin the product. It is important to map the regions of the parameter space at which the flow is unstable. Most of the stability analyses of two-layer rectilinear flow consider the position of the interlayer as an independent parameter. Classical results cannot be applied directly in coating flows. We present a linear stability analysis of two-layer rectilinear flow considering the flow rates as an independent parameter. The predicted neutral-stability curves define the region of stable flow as a function of the operating parameters. The range of coating operating conditions is restricted further, when the condition for the desirable interlayer separation point location are considered together with the stability condition. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2503-2512, 2010