No, it would be saying that your Fi and Ni are totally equal, but that your Fe and Ne are close.
How do you know it never happens in practice?
I am an IXFX who knows an IXXX, two ESFXs, and an XSFX who says one of her friends is an INFX. But I know it's just my word against your proclamation that it doesn't happen. So tell me what you'd like from me.
It makes more sense if you think of it like this:
If you're borderline on a trait you don't just get the abilities associated with both types. each of the functions has characteristics of all of its placements. For example, having Fi and Fe as a first and second function, respectively means I focus on both of them; but also having them as a fifth and sixth function means that I feel confident neither in my ability to establish relationships nor my ability to interact socially. In the same way, my Ni and Si are limited in their individual capacities. I'm not very good at predicting future events, nor am I good at creating relaxing states for people. i am, however, good at understanding the conceptual structure of things, or how the qualities that they suggest (Si) and the interactions between them (Ni) generate an overall essence or vibe. I've also seen Se and Ne at work together, in my XXFP and XXXP friends as well as the EXFP musician Amanda Palmer among others.
And I guess I'd attribute this to having all of my introverted functions come before my extroverted functions in the cognitive charts, but I have a very poor sense of external reality. I have trouble generalizing; according to my family, I lack common sense; and I need to assess the conceptual structure of things to understand them. So basically, being borderline on traits makes the traits that aren't borderline more pronounced.