Recent advances in rheological models for monodisperse dense granular materials are exciting. However, they do not account for the effect of local particle size distributions on the rheology mixtures of particles. It is well-known that particulate mixtures tend to unmix, and their rheological properties are dependent on species concentration. Typically, expressions for the rheology of dense granular flows are explicitly dependent on particle size. However, there is no indication of what may be used for a representative size in a mixture of particles of different sizes. We find, in the absence of gravity, plane Couette cells present an effective geometry for investigating the rheology of binary mixtures of different sized particles. Unlike the behavior of more sparse systems, we find that the dense systems do not segregate much, indicating the usefulness of the geometry for studying the dependence of the mixture rheology on particle sized distribution systematically. In our preliminary studies we find that the pressure at the boundary has a skewed probability distribution function (pdf). We also find that the pdf of the boundary pressure for a particular mixture scales according to the inertial stress.