A lot of people here posted valuable information. Yes both foods don't digest at the same rate, and more importantly different digestive enzymes are needed to digest either food so more energy is used in trying to complete the digestive process.
The prime reason however to never mix raw food with kibble, is because ALL kibble is flavoured with rendered fats. These fats are located on the outside of the kibble and are very very very subject to bacteria growth, so if you had some kibble laying on raw meat in a warm environment, it wouldn't take very long for the kibble to start to grow unwanted bacteria.
Same holds true with moisture. If for any reason your kibble gets exposed to moisture/ water overnight or for any great length of time, toss it.!
Naturally when the dog becomes ill, it's the raw meat that will get the blame....
not the kibble! So please always avoid mixing the two together.