We develop an approach to coupling between viscous flows of the two phases in porous media, based on the Maxwell-Stefan formalism. Two versions of the formalism are presented: the general form, and the form based on the interaction of the flowing phases with the interface between them. The last approach is supported by the description of the flow on the mesoscopic level, as coupled boundary problems for the Brinkmann or Stokes equations. It becomes possible, in some simplifying geometric assumptions, to derive exact expressions for the phenomenological coefficients in the Maxwell-Stefan transport equations. Sample computations show, among other, that apparent relative permeabilities are dependent on the viscosity ratio; that the overall mobility of the phases decreases compared to the standard Buckley-Leverett formalism; and that the effect is determined by the parameter determining the "degree of mixing" between the flowing phases. Comparison to the available experimental data on the steady-state two-phase relative permeabilities is presented.