We show the equivalence of the 2-director and the 3-director continuum theories of biaxial nematics, thereby extending the 2-director theory to the compressible case. We calculate the viscosity coefficients in terms of order parameters within the framework of the 2-director continuum theory. Finally, we extend the Doi molecular approach to the biaxial case and calculate the 2-director viscosity coefficients in terms of the order parameters, diffusion coefficients and temperature.