Autodesk Simulation CFD has the ability to handle multiple fluids in one model. Note that fluids with different fluid materials cannot contact one another unless one or more is a distributed resistance. Non-distributed resistance fluids can be connected thermally (separated by a solid material).
Examples where this is useful include an air-water heat exchanger or flow blown over a sealed electronics component box. In the latter example, natural convection might be important inside the sealed box, and forced convection may play a role outside the box.