We argue that the possibility to observe a stable biaxial nematic phase in a binary mixture of prolate and oblate hard particles is seriously limited by the existence of entropy-driven demixing. This result follows from a simple Onsager-type density functional theory. An important feature is the coupling of the demixing mechanism to the orientational order of the system. The strength of this coupling is dependent on the asphericity of the particles, and is directly related to the stability of the biaxial nematic phase.