@par6
Product Attributes. For example a checkbox that the customer checks for "expedited" (before he clicks 'Add to cart').
Yes, it "actually calculate a box just for the expedited items together and another for ground i.e. just 2 separate boxes".
This is an very old blog (and also a bit more complex scenario), but you can see the basic concept: Packing has an "exclude" expression. So you can first exclude items that are "ground" (i.e. keep just expedited items), get rates from UPS, using the Name Expression to only keep the expedited method/rate. Then Pack again excluding the expedited items, get rates from UPS, using the Name to only keep the ground method/rate. Of course, if you are offering three methods (Ground, 2nd day, Next Day), then you'll need three Pack/Option pairs.
However, although SD can do it, this can be confusing for the customer. First, you might have to put the "expedited" product attribute on EVERY product. And, then you have to consider if you want to show the customer multiple methods on the shipping method selection page. (I.e. whether or not he selects "expedited" on the product page for any items, does he only see one method "Shipping", or can he still choose from different methods"?)