Brushes made up of mixtures of polymers are modeled using mesoscale simulations under shear and the viscosity and the friction coefficient are obtained as functions of the chains' grafting density, brush composition, free chain concentration, and solvent quality. We find that brushes made up of mixtures of polymers can reduce to a greater extent the friction coefficient in comparison to a mono-chain brush and that both the viscosity and the friction coefficient obey scaling laws that depend only on solvent quality. These results shed light on microscopic mechanisms that affect viscosity and friction in polymeric brushes, which are findings of interest for research on nanotribology.