if the interior faces are 'hard" surface zones, i. e. visible in boundary conditions, then the solution is simple: 1) change the interior zones to walls, which will create wall and wall-shadow, and 2) decopule them using the TUI command:
mesh modify-zones slit-face-zone
This will decouple the wall and its shadow into uncoupled walls. Then you change them from walls to interfaces.
All these operations are done in Fluent itself
Hope this helps
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