The appliance may not use two independent thermostats, and the symptoms do not establish the required components. Its control design must be identified and diagnosed correctly.
What the observation tells you
Replacing guessed parts can waste money and leave the actual issue unresolved. A shared fault, loading condition or door problem may present differently across the compartments. Compatibility also depends on the exact model and production version.
A useful next step
Provide the observed pattern and any professional findings first. Have the required work confirmed before purchasing parts, and keep electrical and sealed-system work within an appropriate qualified service.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.