Standard phase-shifting interferometry (PSI) generally requires collecting at least three phase-shifted interferograms to
extract the physical quantity being measured. Here, we propose a simple two-frame PSI for the testing of a range of
optical surfaces, including flats, spheres, and aspheres. The two-frame PSI extracts modulated phase from two randomly
phase-shifted interferograms using a Gram-Schmidt algorithm, and can work in either null testing or non-null testing
modes. Experimental results of a paraboloidal mirror suggest that the two-frame PSI can achieve comparable
measurement precision with conventional multi-frame PSI, but has the advantages of faster data acquisition speed and
less stringent hardware requirements. It effectively expands the flexibility of conventional PSI and holds great potential
in many applications.