haha, whatever!
That point is of utmost importance because mixing in the stomach is a natural way which has happened in our animals and hominids ancestors for millions of years. Eating foods one by one in sequence (as they are usually not found in the same place and at the same time) allows mammals to properly feel how much of each food they need.
On the contrary mixing before ingestion is a completely new method which in most cases requires some tools such as pottery — and pottery is one of the most distinctive features marking the Neolithic separation from Paleolithic. Our adaptation to such food mixtures is certainly very incomplete, as we can easily experiment. For example, mix a few drops of lemon juice with oysters and the smell of lemon will completely mask the oyster’s own smell and deeply change its taste. It’s an efficient way to get poisoned by a bad oyster!
I suppose you always monomeal, right? 
No, not necessarily and currently most of the times not. A few people do and I tried their method to eat several mono-meals per day, but I ended by eating always until late in the night. It deeply disturbed my habits and well being, so now I try to eat only twice a day, once around noon and once in the evening, as I and most other “instinctos” have done without problems for several decades.
Unless we are totally satisfied of our meal with a single foodstuff, there’s no reason not to search for and choose a second food, then perhaps a third and a fourth. Some food can bring enzymes helping digestion of the first while some don’t digest well together. This is something we learn by experimenting. What is important is not to eat again some more of the first food after we had felt an instinctive stop with it and ate something else – which would allow us to like the first food again because of the enzymes brought by the second, this resulting in an overload of some substances.
Usually, sweet fruits digest well with other sweet fruits, vegetables with other vegetables (including tomatoes, red pepper, avocados and cucumbers which are botanically fruits). Significant amounts of nuts don’t digest well with other nuts and avocados. It’s also wise to limit the intake of animal foods to one kind of animal per meal, as it would naturally happen in nature. Vegetables can generally be eaten without problems shortly after (or before) meat, fish or eggs.
Ok, I ll be clearer with my combos:
peaches and onions : one mixed meal
Must be horrible, wasn’t it?
mandarines and walnuts: 2 mono meals within a short period of time (maybe 40 min)*
Not ideal, but it depend on your actual state, hunger and respective amounts.
banana and walnut: one mixed meal
Prevents you instinct to properly dose each.
