In my experience with limited diets, it is always advisable to take in as much variety as possible. So even if I only had milk and potatoes, for example, I would scavenge for some edible greens to add. Nutrition is not just about amino acids - there is a whole range of vitamins, minerals, and other less well known substances in foods that keep us healthy. I think if a person is down to having only one of your listed parings available to eat, they have a lot of problems to solve!

I lived a number of years in the rural Dominican Republic, and befriended some very poor Haitians who migrated there illegally. They subsisted on rice and beans to a great extent, but substituted a variety of other starch vegetables (green banana, various root vegetables, bread) when they could - and also learned through community collaboration, which wildly growing green plants were palatable. They would also sometimes buy the local “sausage” which contained about 10% meat byproducts, and the rest was mostly rice with probably some corn and other grains and spices). My point is that nobody reading this is ever likely to be as destitute as an illegal Haitian living off the land, but they still ate a somewhat varied diet.

The thing they don’t mention when they say that a combination of food items are “nutritionally complete”, is what quantities they need to be consumed in. Some micronutrients are technically in a lot of foods, but you’d have to eat the equivalent of 15,000 calories a day to get what you need. There’s definitely a lot of people around the globe who get by on rice and beans for 90% of their diet, but they do have to eat other stuff. Lots of nutrient deficiencies also take a while to start to have an effect, and the effects might not be obvious. I know what pellagra, scurvy, and rickets do, but I have no clue what a selenium deficiency would do