A cooperative buyers’ app

Say you and your spouse is shopping food together. You are both running in the same store and looking for the same goods. You have to check in with each other often to sync what each have taken and what you should continue with.

Instead there could be an app, or simple web site, that does the syncing.
Let’s say you have a shopping list stored in your phone. You give it, or you are given, a unique id. Give this code to your spouse.
As soon as you check any item on the list a signal is sent to the other phone, directly or over internet, to check the item on other phones using the same code.
That way you can go shopping together without having to go back and forth to the shopping cart to discuss.

Extra bells and whistles could be the ability to send small messages like “double the milk” or “do we have eggs at home?”