Hi,

chtMultiRegionFoam can solve for multiple solid and multiple fluid domains (assuming the two fluids are not a multiphase field, but rather two distinct disconnected regions). Have a look at the mutliRegionHeater tutorial test case to see how it's set up (tutorials/heatTransfer/chtMultiRegionFoam/multiRegionHeater).

As an alternative, you could also use preCICE (https://precice.org/) to couple the different OpenFOAM regions. preCICE is a generic multiphysics coupling library.

While both routes have a fairly steep learning curve, I've always found the preCICE route to be slightly more generic, easy to customise, and intuitive to use, once you get the hang of it.

Regards