They're always paired in any given type. If Te is present in the top four, so is Fi. If Ti, then Fe. If Ne, then Si. And if Ni, then Se. (And vice versa on all of them.)
I wonder, do these pairings represent two functions together, or a person's exaggeration--her unbalancing of a balanced thing--via preference. That is, what if Te and Fi, or any of the other pairs, are not pairs, but instead they are exaggerations, distortions, extreme perspectives placed on things that, somehow, are the same thing?
What same thing? Dunno.
But why suggest the pairs might somehow be not pairs but somehow "the same thing"? Because I look around and it seems to me that inferior and tertiary, but particularly inferior, functions have an essential role in the life of the top two functions. Like, it seems that Ni really can't exist without Se. How could it? With no information coming in about what things look like moment to moment, how is there anything at all for Ni to sit back and process? Likewise, Ne and Si, if there is no library of at least some basic steadfast unchanging well known and detailed information, how is Ne to propose its wild possibilities? And maybe likewise Te+Fi and Ti+Fe, but I haven't thought how to make that seem plausible yet.
But so what this would suggest is that, once again, you don't mix and match functions. You get judgment and you get perception, and that's it. And for some reason, you're born wildly unbalanced in which aspect of judgment and which aspect of perception you prefer.