[Linse]

Dear Zahra,

for chtMRF basically it is the same principle as for the cases above:
If you have fluid flowing through this internal interface, it should not be split into different regions but remain one flow region. Even if you combine different parts during meshing, these internal interfaces should not be defined as boundaries.
If in contrast you are thinking of two regions of immiscible fluids, just let splitMeshRegions do its magic and set the wall conditions.
If you are trying to use different flow settings in the two different fluid regions and there is supposed to be full communication between them: I have worked on some interfaces for such a case some time ago, but that work is not yet finished - although I plan to revive it during May and June. To my knowledge, so far there is no such interconnection-condition in the official release yet.

But if you just want to define e.g. two different temperatures for the fluid "regions" I suggest not taking the way of defining different regions but using the setSet- or topoSet-functionality. Definition of the cellsets should be working similar to the region-setting, but you will not have to partition the mesh for the different fluid zones. As an example for what I mean you could look at the damBreak-tutorial, I think there it is put up in a way easy to follow.

Cheers,
Bernhard

Edit: If you want to consider multiphase flow within chtMultiRegionFoam I am quite certain you would have to do some programming in the solver...