In this Letter, a hybrid interferometer for simultaneous measurement of the partial pressures of O2 and CO2 mixtures is reported. The sensing head consists in the combination of two interferometric structures, one a Fabry-Pérrot cavity and the other a modal interferometer. The intrinsic Fabry-Pérot was formed by splicing a single mode fiber with a graded index fiber length that was then subjected to chemical etching creating an air cavity. The second interferometer is based on a splice of a pure silica tube in series with the Fabry-Pérot. It was observed for a particular gas that its refractive index changes with pressure variation in a specific way, a characteristic that permitted the simultaneous measurement of partial pressures of CO2 and O2 with rms deviations of ~±48.7 kPa and ~±20.1 kPa, respectively.