Although I am not sure if this is a good explanation or not, but I think it is worth putting out there. Well, here it is:
Each function deals with reality differently, possibly due to circumstance of one’s own self appearance. As Waddles pointed out, there is a bulk energy flow to a part or parts of the brain that becomes shaped and transformed into information that manifests itself into what we call a function (or in another words, a function receives energy and transforms it into a message and/or behavior). This could be a sign of a specific gene or set of genes, which has led me to speculate about the evolutionary development into socionics.
Has any one heard of facial symmetry and so called developmental stability? Is it possible that duals create “developmentally stable” children? As everyone knows by now, I think it was possible that mankind before a point of development acted more on a subconscious level. In which case finding one’s dual would have been more automatic and energy saving since people would have acted more automatically and would not have to resist to biased viewpoints that we see in society today. So there may be evolutionary reasons why the human personality exists in the way that is now identified as socionics. Therefore it is possible that duality is a sign that genes complement to that other person.
This of course is just pure speculation; I only wish that this could be proved or disproved by hard evidence.
On another note, if this were to be true, then it would be interesting to see how it had evolved that way.