This article was taken from a Raw Feeding distro list with permission...
There is nothing neutral about a dog's stomach contents, no matter how much grain/carbs you feed. If you've ever seen a dog vomit up bile, or even food, even exclusively kibble fed food, onto a metal crate pan, you would have seen "proof" - the contents will etch the galvanized metal!
Dogs were "designed" to eat meat and bones, and perhaps a little bit of plant material - being carnivores. They produce a strong gastric acid, somewhere in the range of 1-2 on the pH scale (neutral is 7). In comparison, humans, who are omnivores and are "designed" to eat a greater variety of food but under different conditions, have a pH average of 5. This is the reason why humans would quickly become very sick if they ingested the level of bacteria dogs are able to tolerate. I think most people know that commercially made white vinegar is a good agent to clean and disinfect with, because it inhibits the growth of bacteria - vinegar is about 2.4 or so on the pH scale.
Thing is, acid isn't the only thing involved in digestion - there are also enzymes. Carnivores are not designed to process carbs (grains, etc) efficiently because they do not make the appropriate amounts or type to digest them - omnivores have the appropriate enzymes to do this. Dogs CAN digest plant materials, but they do so very inefficiently, and often incompletely (making the nutrients not always bio-available to the dog, despite being fed and IN the dog's gut). If the dog is fed a plant based diet, for example as with some kibble, their digestive systems can be "trained" to produce more of the enzymes needed, but it's never what is optimal, or even acceptable, which is why many dogs end up with a very large volume of stool. Some of it is indigestible fiber, but a lot is undigested plant material because it's not possible for the dog to to so.
There was an old study done on digestion rates when fed different foods to determine stomach emptying time for administering anesthesia. Raw meat was fastest, kibble slowest (and what remained of kibble in the stomach a day later was putrid, if I remember correctly) - I am trying to find either a reference or the text of it but haven't yet. Prolonged and incomplete digestion may be relevant to problems like bloat (which is why many breeders of breeds with bloat issues choose to feed raw exclusively), but in general, most dogs manage to get rid of all sorts of unsavory stomach contents without incident, although some do it with a great deal of excess gas.
The mixing issue - such as it is - is not so much from the actual mixing. The dog will digest - eventually - what's in the gut, and in most cases no apparent problems will occur - the strong acid takes care of that. That's why many people feed all sorts of combinations and never have any problems... Until they do. Problems can occur if the dog is under additional stress, so that it's not digesting "up to standard", if it's old or young and it's digestive system is not working up to par, or if the bacterial load is particularly heavy and overwhelms the gut. Depending on the degree of the problem, and the specific bacteria, there may be a transient problem with loose stools or a prolonged, serious illness. So, basically, you CAN feed whatever combo you want, likely without problems, but there is a risk involved. Separating raw from kibble helps to reduce the risk.