With the mixture theory of Bowen, and some assumptions, it is possible to derive solutions. For example, a Bernoulli law for the two-phase flow of entire mixture will contain contributions from diffusion velocities. Assuming one density larger gives a simplification such that only one diffusion and the you can solve the Bernoulli law as an algebraic equation for the velocity. It will be a function of the velocity of one phase. For that you can assume your known solution, but then you neglect the momentum supply.
There are also other two-phase flow theories. In some they assume the same pressure for the two phases, but that is not mandatory.
otherwise, in my opinion you need experimental results. two-phase flow is complicated and unpredictable unless the geometry is well known and extensive experiments have been done in order to validate any proposed model.
The following papers may be found helpful: https://link.springer.com/article/10.1007/BF00614812 https://link.springer.com/article/10.1007/BF00714688
For a valuable classic book reference, see: Jacob Bear, "Dynamics of Fluids in Porous Media", Dover Publications Inc., New York, 1988 (corrected republication of the original ed.: Elsevier, New York, 1972); cf. Ch. 9: "Flow of Immiscible Fluids".