There is absolutely no reason to include kibble in a raw diet but there is every reason to include raw in a kibbe fed diet.
Yes, they do digest at different rates (raw faster in my experience with foster dogs in poor health). That said, broken down food (aka chyme) doesn’t “sit” in the stomach while the remainder of the food continues to break down. It moves on while leaving undigested food to continue digesting.
I agree with Crazy4cats, if you look at how food is actually digested, adding raw to a kibble diet is going to enhance the digestion of the kibble.
