Shopify has two ways of blending rates from multiple profiles:
Based On Shipping Method Title
If there is a rate within both shipping profiles that has the same name (e.g. Standard Shipping), Shopify will blend these two rates together to display a checkout.
Based On the Cheapest Cost
If rates from multiple profiles have different names, Shopify will take the cheapest rate from each profile and blend these costs together with the title Shipping.