Each meal and cavity has its own mode, load and cooking requirement. A shared start time does not make their completion conditions identical.
What the observation means
One may have been preheated, while another programme expects a cold start. A small covered dish and a large roast also place different demands on their respective cavities.
A useful next step
Follow each recipe and cavity's instructions independently. Record an unexpected change against that oven's previous normal use rather than comparing unrelated meals. If a shared timer controls both ovens, confirm its operation before relying on one alarm to end every cooking process.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.