We propose a new electron backscatter diffraction (EBSD) indexing approach that can handle overlapping patterns arising from fine-scale mixtures of two crystallographically distinct but related phases. The approach relies on a combination of spherical indexing (SI) with an overlap master pattern (OMP) in which the two phases have the proper orientation relation. We illustrate the approach using a Ti-10-2-3 sample and compare results from traditional Hough-based indexing of the α and β-phases with SI maps. We show that the SI approach produces superior orientation maps, and we present a symmetry-based algorithm for the determination of the α-phase variant when experimental patterns are indexed with respect to the OMP. We illustrate the use of the OMP to determine the parent β-phase volume fraction and obtain excellent agreement with Thermo-Calc phase fraction predictions.