Merge shopping cart of two different sites

Hello all,
I have a question.
I want to make two different sites. Before I come to the payment screen on these sites, I want to make a link to the other site. For example, “You can combine your purchases from http://www.domain.com before completing your order.” like…

As a result, the customer will make an order from both sites and pay a single shipping fee.
Is there any way you can do this? I would be glad if anyone could give information.