In shallow water environments, the swath coverage of a sidescan bathymetric system is ultimately limited by the presence of multipath interference. A sonar design combining multibeam and sidescan technologies offers extended swath coverage by suppressing multipath contributions. Synthetic data generated with realistic computer simulations are used to study the performance of the proposed hybrid design in an environment including multipath interference and ambient noise. Phase measurements were made within multiple pairs of spatially separated beams. The widest possible bathymetric swath is then produced by combining uncontaminated data from all beams. It is shown that this design can offer a significantly extended bathymetric swath in multipath environments, compared to conventional bathymetric sidescan systems.