I know there´s the solo-checkout function. BUT this is different.
For a specific product, be able to choose which products can NOT be added to the cart, when THIS product is already in the cart.
Let´s say someone does a checkout for product A, goes to a thank you page where you want to offer another product(s).
Customer doesn´t decide straight away, clicks a bit more on things on the website / reloads pages… and before you know the product which he already purchased is in the cart as well.
So, I think it would be great to be able to set in the product settings, which product(s) can NOT be added to the cart, when the current product IS already in the Cart.