The two infrared (10‐&mgr;m) wave fronts of an interferometer (Michelson or Mach‐Zehnder) interfere separately with a tilted reference beam on a dynamic recording medium consisting of a thin oil film on a glass substrate. Illumination of this twin hologram with a visible (He‐Ne) beam reconstructs simultaneously, in the same direction, two wave fronts characteristic of each arm of the interferometer. Practical realization and experimental constraints, such as air turbulence and mechanical vibrations, are easily satisfied with these long‐wavelength interferometers. Experimental results include air wedge determination, homogeneity test of a NaCl plate, step deposit evaluation and afocal assembly analysis. The real‐time aspect of our recording medium also permits the visual observation of transient phenomena.