Physical processes ruling bed load transport are poorly known, impairing our ability to compute sediment flues in rivers especially on steep slopes. We present an experimental study of two-size mixtures of coarse spherical glass beads entrained by a shallow 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 12.5%. Water flow rates and solid discharges were kept constant. They were adjusted to obtain bed load equilibrium, that is, neither bed degradation nor aggradation over long time intervals. Using image processing algorithms, the trajectory of each particle and its state of motion (rest, rolling or saltating) were determined. Comparison with previous results confirm that the collective motion of rolling is specific to particles of uniform size whereas the saltating regime characterizes the two-size mixture. The use of a two-size mixture resulted in a bed mainly formed by the 4mm beads. To understand the downward migration of the fine particles, an equilibrium flow with only 6mm beads and a very small rate of fine particles was also analyzed. For the migrating beads, the transitions between rest and rolling were principally caused by a neighborhood of four large beads. This is due to the loose packing of large beads favouring the downward migration of fine particles.