A better way to look at it is that there is the single function, Feeling, and different types will have different perspectives that prefer either an internal or external orientation of it.
Type is determined by the preferred perspectives, not really "using" one function more or less in comparison to another.
So en ENFP will prefer an internal orientation of Feeling, and it can become ambiguous ans they are really an extravert through their dominant intuition, so with the E and F in the code, they are "extraverted" and "Feeling", so when engaging both preferred functions (N and F), they might in some way appear to be "using" extraverted Feeling. But if they are really FP, the internal orientation will ultimately be preferred as the standard of their values, regardless of how much it might seem to get externalized.
Only as much as you can you enter a room at the same time as coming out of it. Like a mentally unbalanced person might be able to do that.
I think the function theory is describing a healthy person and says that a healthy person uses either Fi/Te or Fe/Ti but not bits of both.
It's like the Fi/Te person follows the groups mentality and their own feelings while the Fe/Ti person follows their own mentality and the groups feelings.
If a person were to start using both Fi and Fe equally then since they would be trying to go with both their own feelings and the groups feelings that could cause a big conflict in them and scramble their brain a bit and since they wouldn't be using logic in either their external or internal world they will go crazy. I tend to think the brain is wired to go one way or the other. It can go the other way but that causes problems and happens in times of stress when the shadow functions come out to play.
If they used only Te and Fe, then they would be just a mindless robot doing and feeling what people tell them.
If they used only Ti and Fi, then they might start disassociating from reality and people and become rejected.
If they used only Te and Ti , they will become very hard and cold, making logical decisions with no feeling.
If they used only Fe and Fi, they won't be doing anything logical since they act only on feelings and become crazy.
I think Te can look like Fe such as in the instance where you value feeling peaceful. Te may reason if you go around insulting people it will destroy any feelings of peacefulness. And so you treat people nicely and do Fe type activities.
I don't know, I guess it depends on the person, and I would be inclined to say that the answer as to whether a person can use Fe and Fi equally is usally no.
As an ISTJ, my tertiary function is Fi, and I think I use it a lot. But I think I have learned to use a lot of Fe as well (even if that's not generally a function ISTJs use, I know I have picked it up through nurturing. . . I think my Mom's an ISFJ or ESFJ). I don't think I'd be using Fi and Fe simultaneously though. Different situations would bring out my Fi, and different ones would bring out my Fe. I think I use more Fi than Fe though.
I guess you can make an effort to develop the feeling function that is not common to your type and start using it. But it won't be as natural.