In this article, residue curve maps (RCMs) are developed for two-membrane permeators, and it is shown how the topography of the resulting maps can be modified by the manipulation of certain operating variables. In particular, it is noted that in a three-component mixture for which two membranes have inverted fast- and slow-permeating components, it is possible, under certain conditions, for the shared intermediate component to become the stable or unstable node of a two-membrane permeation system. Such a configuration is simulated as a continuous process, and performance improvements are noted when it is compared to a series configuration of conventional permeators using those two membranes. A methodology for the synthesis and preliminary design of two-membrane permeators is developed. To illustrate further the methodology developed, brief discussions are included illustrating the application of the method to ternary systems exhibiting complex behavior, and a four-component system consisting of nitrogen, argon, krypton, and xenon using membranes of either 4-methylpentene or silicone rubber with cellulose acetate.