Hi,
I have a current Magento 2.4 installation and I want to have another store with different categories and products but share customer information, cart, checkout and payments. I want the user to be able to purchase something from either store at the same time, then checkout using a single cart and have the order confirmation email to go to each different store owner.
Is this possible by using either multiple websites or multiple store options?
Out of the box without any extensions or custom code - almost. You could do everything you described except combine multiple products from different websites into a single cart.