How do we shop at two stores without buying things twice?

Assign one owner before either trip begins

Write the item and the responsible shop together: “market — peppers and bread,” “supermarket — pasta and soap.” Both people can still see the whole list. An item without an assigned shop remains open for a decision; it does not become permission for both people to buy it just in case.

Pass a shortage across explicitly

When the first store is out of something, mark it as missing and tell the second shopper before moving it across. Checking it off normally implies it was bought. That small difference matters when both trips happen at the same time and neither person can see the other's basket.

When the market has no peppers, the market shopper can write “unavailable here; can you take peppers?” The supermarket shopper needs to answer before the first person treats the item as transferred. That matters if the supermarket shopper has already passed produce or no longer has room in their bag.

Reconcile the two baskets once

At home, check the short list of unresolved items rather than comparing every receipt line. If peppers came from neither shop, choose the meal change or next trip. If both shoppers bought them despite the handoff, record where the communication failed. Changing the ownership rule is more useful than adding more notifications to both phones.