Find the ingredient both people actually want
Ask each person which part of an ordinary dinner they would happily eat again. They may both like potatoes but disagree about sauce, or both enjoy noodles while choosing different vegetables. Build the meal around the overlap rather than treating one person's usual dinner as the default everyone else must modify.
Choose where the dishes split
Check how much extra work the arrangement creates. Two toppings are easy to manage. Two separate sauces with different cooking times may be more effort than cooking two different dinners. Let either person choose an entirely different meal on nights when the shared base has little appeal.
Draw an imaginary line at the last step both plates share. A bowl of plain rice can serve everyone before toppings go on. If one serving needs a different base, the split happens earlier. Check that each person gets a complete dinner and that the extra work has an owner.
Try the arrangement for one meal
After dinner, ask what each person would change. If one plate needed substantial extra cooking or the shared base was left untouched, try a different overlap next time. Keep a successful combination as a named dinner so it remains easy to choose on a busy night.