When creating 2 checkouts from 1 cart, a new cart is being generated.
What is happening:
Items are added to the cart, a storefront request is made and a checkout url is generated. The checkout url is then rendered, but not completed.
If you then navigate back to the storefront, add another item, generate a new checkout and render the checkout Shopify will create a new cart (losing reference to the prior one).
I saw the new createCart mutation and can use this to get around this limitation, however, I am just interested in how the cart/checkout interaction work since I could not find this in the documentation.
*Edit: I am pretty sure this was happening because I was going over the storefront api rate limit.