Global Shopping Cart not Working as Expected

I was hoping that MarketPress would allow me to create a network of sites that all share the same shopping cart/checkout.

What I would like to have happen is a user visits site1.com and adds a product from that site to his/her cart, then goes to site2.com and is able to see the item from site1 in their cart and is able to add another product from site2 and then proceed to checkout and pay for both products from site1 and site2 in one shopping cart.

I have mapped some sites in a network and I am able to display the global product list, but the shopping carts do not transfer from one site to another. When you add a product from site1.com it does not show up in site2.com's shopping cart and vise versa.

Is this functionality supposed to be supported by this plugin?