I know at least one person will be thinking "because otherwise socionics wouldn't work" or "so that Model A works properly", but I want to know why we value Fe/Ti or Si/Ne and cannot possibly value Fe/Te or Si/Ni. One possibility is that we can't physically use two extraverted functions; for example, we can't use Fe and Te, because these two functions concentrate on the same external reality but in different ways, and it is impossible for a human being to do this i.e. they must concentrate on external reality in a single way. I might've answered my own question here, but please add your thoughts.