Use a matched pair of dinners
Choose meals with different demands that fit either of the uncertain nights. A simple dinner needs to work for the person home alone. The meal for two should be one people actually want to wait and eat together. Avoid pairing two elaborate dinners and assuming the shift change will somehow create more time.
Name the point when the pair swaps
When the shift is confirmed, move the meal names rather than rewriting the whole week. Before buying fresh ingredients, check whether both meals still fit the new timing. Keep the decision visible to whoever is shopping or cooking first.
Write “check Wednesday rota by Tuesday at noon” beside the two meals. Whoever receives the update tells the cook and shopper directly. A new time on a work calendar may settle the shift, but it does not automatically tell the household that tonight's dinner changed.
Make the late answer manageable
If neither evening is confirmed by the shopping trip, choose ingredients useful in both meals or give each dinner an acceptable alternative. Keep the extra cooking for the night with enough time. If the shift changes twice, decide who releases the other person from the old cooking arrangement so they can make their own plans.