Multisite and Marketpress

When you have a multisite and marketpress set up with different users having shopping carts, how can you streamline the checkout process when a buyer buys products from different sellers?

out of the box, you have to check out separately from each seller – and that can be a problem and hassle and confusing since the “view cart” will only display the products from the “seller site” you are currently on.

The way it is done on for example is that the checkout process and view cart displays the cart from each seller on the same page.

Any help is appreciated.