So I just had a thought, inspired by this article, which Gul linked to here. Rick was talking about how using a function for too long, even the base function, will deplete that function's energy resources, after which it needs to rest for a while in order to regenerate and restore it to full power. He also talks a lot about neurons and stuff, but that's not my primary interest.
I have been thinking for a while about another question. It's difficult to describe in general terms, so I'll use an example. Ti and Fi can be thought of as opposites (External Statics of Fields vs. Internal Statics of Fields); it's said in some places that they can't both be active at the same time. However, Ti and Te can also be thought of as opposites (External Statics of Fields vs. External Dynamics of Objects); they're also sometimes described as not being able to both be active at the same time. Both pairs of "opposites" are sometimes described as "two sides of the same coin". However, this can't be true in the same sense for both, so what's the distinction?
Also, DCNH subtype theory (which I know not everyone accepts, but bear with me) posits that if Ti is strengthened, Fi will also be strengthened. At first glance this doesn't seem to make sense: if Ti is being exercised, Fi should be dormant. So why would Fi become stronger? This is also somewhat backed up by the fact that the Base function is the strongest of the Ego functions, while the Role function is the strongest of the Super-Ego (and similarly with the Vital Ring). Both suggest that the two elements composing each temperament (IJ, EJ, IP, and EP) are somehow linked.
The possible explanation that just occurred to me (or part of an explanation, really): is it possible that the two elements composing each temperament (Ti and Fi being the example) share the same energy resources? That is, if you drain your Ti energy pool, do you simultaneously drain your Fi energy pool? Is it in fact the same pool? If only one at a time can draw from the same energy pool, it would explain why only one at a time can be active. If the energy pool grows bigger/stronger with exercise, it would also explain why strengthening one strengthens the other in DCNH.
I can offer one piece of anecdotal evidence, which I thought of just now: when I come home from work, with my Role Fi fried from a long day of constantly dealing with customers (trying to make sure they like -- or at least don't hate -- me, and by extension the store), I find it nearly impossible to sit down and do any kind of hard thinking or analytical work, which I normally enjoy. I have to sit back and relax for a while, possibly watch some T.V. and laugh at some silly comedies. In other words, I have to retreat into my Super-Id and indulge my Si and Fe while my Fi and Ti recharge. Since my job doesn't require a whole lot of Ti, this suggests that my Ti was drained along with my Fi.
Anyway, those are my thoughts. I figured I would post them and see if they spark any further ideas from anyone else. Right now I'm trying to figure out the relationship between Ti and Te, and how exactly they're connected. If anyone has any thoughts, or anecdotal evidence, I'm interested in hearing them!