The two-phase flow generated by breaking ocean waves plays a crucial role in various geophysical processes, including dissipation of wave energy and atmospheric gas exchange. This paper presents a technique to measure the two-phase flow generated by breaking waves at prototype scale. We have demonstrated the validity and potential of this technique in the Large Wave Flume (Grosser Wellenkanal, GWK) facility in Hanover, Germany. Actively breaking, depth-limited waves were measured using an array of three downward-looking lidars mounted above the water surface and an upward-looking multibeam sonar below. This novel setup enabled the characterization of the complete upper boundary (free water surface and splash-up) and seaward lower boundary (entrained cavity and bubble plume) of the breaking wave. We have quantified the migration of the lower boundary as the cavity and plume are entrained in the water column - penetrating toward the seabed, moving onshore with the passage of the wave crest, and then rising as it is slowly advected offshore. We have also estimated the overall composition of the splash, cavity, and plume as the breaking wave evolves over time. Our observations are consistent with results from previous small-scale laboratory experiments and the suitability of the technique for experimentation at prototype scale has been demonstrated.