When you find an item you would like to purchase and click the BUY NOW button, the next page will redirect you to the store the item was added from to "Add To Cart". 

Once you add the first item, you would return back to the registry to add the next item to the cart. If the items are from the same store, you will simply be adding the items to the cart one by one. After adding the last item, you will notice that the others are still in the cart and ready for checkout.

However, if the items you want to purchase are from different stores, there will be a separate transaction placed with each website.