Multi-Site shopping cart

I have a multi site "Mall". Each site is a directory not domain. One main site is the head of the "Mall". Each site has its own products. I need a search box that no matter which site the search box is on, will search for products on all sites.

Also: I can shop from each site, add products to cart from each site, and all products in cart show no matter which site i am on when i goto checkout. This is good. However if i have shopped on different sites and have a couple of products from each site in my cart, when I click on the cart i would like the cart to be located on the main mall site, with the main mall header, not an individual merchant site.

Thank you for your time.