An equimolar mixture of hard spheres with equal diameter d, having an unequal collision diameter d,2 between unlike species, favors a hetercoordinated arrangement if d12 lies between 0.65d and d, and a homocoordinated packing below d12=0.65d. At d12<0.65d, hard spheres of one type move increasingly unaffected by the presence of hard spheres of the other type. This conclusion was drawn from Monte Carlo (MC) simulations over large ranges of density and d12 and originates in the fact that collisions between like-species at d prevent collisions of unlike-species at shorter distance d12d.