There are some restrictions for dual cell model . Please refer this link
Make sure your meshes are one to one paired properly at interface.
48.7. Boundary Conditions Task Page (ansys.com) Please refer to this link on how to set up appropriate boundary conditions.
I recommend you to do mesh manipulation like create the primary fluid mesh and secondary fluid meh seperately then append the meshes one over the the other .
In the case of a heat exchanger in which the primary and auxiliary meshes are not identical, heat transfer may be non-conservative (that is, the heat lost by the hot fluid may not equal the heat gained by the cold fluid). To minimize the difference in heat transfer, the topology and size of the primary and auxiliary cells should be as similar as possible, with the ideal being one-to-one cell conductivity.I hope this helps you in your simulationThanks,