A combined cavity may select one cooking mode at a time. Its grill and oven functions share the physical space and controls described by the manufacturer.
What the observation means
Turning two controls does not necessarily create two independent zones. Some models use a permitted combined function, while others restrict combinations. The manual resolves this rather than a generic range-cooker rule.
A useful next step
Use the approved selection for the meal. Record both knob positions if an unexpected result occurred. Do not force selectors past stops or try to keep a grill energised through a mode that the appliance does not support.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.