Proton velocity distributions in the high-speed solar wind are sometimes observed as two components of approximately equal temperature with an average relative drift velocity parallel to the background magnetic field. This relative drift gives rise to several proton/proton instabilities; for representative parameters, linear Vlasov theory demonstrates that the electromagnetic modes most likely to grow are a magnetosonic instability and an Alfv¿n instability. Two-dimensional initial value hybrid simulations of both instabilities are carried out in a homogeneous plasma. Enhanced fluctuating fields from both instabilities yield a reduction in the relative drift speed and a characteristic heating of the more tenuous component temperature perpendicular to the background magnetic field. Ensembles of simulations yield constraints on the fluctuating field energy density, the relative drift speed, and component anisotropies. Some of these constraints are cast as concise analytic expressions which may be indicators of proton/proton instability activity in the solar wind. ¿ 1999 American Geophysical Union