Bed load transport is a longstanding problem despite its major importance in river morphodynamics. The physical processes ruling coarse-particle/fluid systems are indeed poorly known, impairing our ability to compute local and even bulk quantities such as the sediment flux in rivers. We present an experimental study of two-size mixtures of coarse spherical glass beads entrained by a shallow turbulent water flow down a steep channel with a mobile bed. The particle diameters were 4 and 6mm, the channel width 6.5mm and the channel inclination typically 12.5%. The water flow rate and the solid discharge were kept constant at the upstream entrance. They were adjusted to obtain bed load equilibrium, that is, neither bed degradation nor aggradation over sufficiently long time intervals. Flows were filmed from the side by a high-speed camera. Using image processing algorithms made it possible to determine the position, velocity and trajectory of each spherical particle as well as fluctuating solid fluxes and flow depths. Transitions of the state of motion (rest, rolling or saltating) were also determined. These new data were analysed and compared to previous results obtained with spherical particles of uniform size. The use of a two-size mixture resulted in a bed mainly formed by the 4mm beads because of downward migration of the fine particles. The movement of single particles was studied taking into account their neighborhood. The transition between an equilibrium flow with only 6mm beads and the flow with two size mixtures was also analysed.