But just to be clear, whey is fast meaning an hour to fully digest. It's not like it takes 5 minutes or anything lol.. Comparing whey to casein is like comparing a turtle to a snail.
I have heard about the carbohydrates slowing each other down, but never proteins. In reality for me, I take a 60 gram whey shake immediately preworkout, so I'm getting a good steady stream of aminos anyways. I'm just not sold on whey/casein slowing down digestion, but I do understand your outlook on the blending in the stomach.