Fabry–Pérot refractometry is a leading technique for high-accuracy pressure realization; however, multigas operation is typically constrained by gas-handling complexity and the risk of cross-contamination. In this work, we investigate whether the pronounced difference in time scales between rapid pressure equilibration and the much slower molecular diffusion in long, narrow connections can be exploited to enable two connected Fabry–Pérot cavity-based refractometers to operate with different gases at the same pressure. Diffusion modeling based on Fick’s second law, together with experiments using two refractometers, shows that gas mixing can remain below 1 ppm for a variety of measurement times and pressures. These results demonstrate a simple and robust approach to simultaneous multigas refractometry at a common pressure without the need for physical separation hardware.