The enzymes essential for digestion only function at specific pH levels; proteins and carbohydrate enzymes require opposite pH levels. If you mix them together in the same bag, you won't be able to digest either food properly.
"Most fruits go straight through the stomach into the duodenum for digestion, which means if you put fresh fruit on top of a big meal, it has to sit and wait in the top of the stomach until the other food is digested, during which delay bacteria attack the fruit and ferment it, gobbling up all the nutrients and leaving you with gas and metabolic wastes."
Daneil Reid, Trophology: The Science of Food Combining, "The Tao of Health".
In 1908 Dr Howard Hay after practicing medicine for 16 years developed High Blood Pressure, Brights Disease and a dilated heart. As there was no treatment available, Dr Hay decided to find a cure for his own symptoms. He looked at his food intake and the chemical process of digestion, the enzymes that are essential for this process and whether the food has an overall acidifying or alkalizing effect on the body.
To the amazement of his colleagues, he had a complete remission of his symptoms in 3 months of changing his diet. He also reduced his weight by 20kg's. In 1911 he introduced his Food Combining guidelines and gave lectures till his death about the need to combine foods properly and reducing acidifying foods from the diet.
If you think there's nothing to acid-alkaline food combining, try this: one day for lunch eat a steak with chips, and monitor how you feel after an hour or so. Next day eat the same steak, but with a large vegetable salad. Which day did you feel more tired, drowsy?
Try feeding your dog meat and biscuits for a while, see how he feels!
Try an acidifying McDonald's diet for a month.
If you're still alive, try an alkaline diet of alkaline water, fruits, vegetables, juices and super greens for a month. You're guaranteed to lose weight from this diet - it's over 95% water!