The other parent serves different food
Agree on allergies and the diet first. Those are the rules that do not bend between houses.
One food that always works
The familiar foods can differ a little by house. Tell each other one food that always works, so neither house has to guess at 6 p.m.
Rice at one house and potato at the other is fine. Cheese at one house is not fine if milk is an allergy at both.
The rules that do not bend
Milk means no cheese and no yogurt. Egg stays off when egg is an allergy or the diet is vegetarian. Wheat means no bread, pasta, or oats. Meat stays off when the diet leaves it off.
Write the sentence down. A text is better than a memory during a handover.
Different plates, same safety
You do not have to serve identical menus. You do have to serve safe ones.
If a reaction happened at the other house, both of you stop that food and talk to your child’s doctor. Do not retest it at your table.
A sentence you can say
“Allergies first. One food that always works.”