This is a bit of a weird question, I guess. I know how they work on the surface. Endurance regenerates quickly, health stays gone until rest.

But I'm really not sure how the damage is distributed among them. Does it go to both of them at the same time? Is it a percentage? Sometimes it seems like a character loses a lot of health, but their endurance is untouched, or almost so - and losing as much endurance as they did health would send them straight into unconsciousness.

So, anyone knows how it works from a technical perspective?

Health and Endurance together make up a character's vitality in PoE. In a nutshell, Endurance is a short-term measure of vitality, meaning how much punishment can be sustained before a character is knocked unconscious in battle. Endurance can be replenished on the fly, even in battle, by spells, potions etc., and replenishes of its own accord outside of battle. Health, on the other hand, is the corresponding long-term measure of vitality, meaning how much punishment can be sustained before a character is maimed or dies permanently. Health does not replenish of its own accord, and is more difficult to restore, this being done usually by resting. A character has more health than Endurance and when their health is depleted they become maimed. If a character sustains damage whilst maimed they die permanently.

Yes, I know that. I understand how they're replenished and what happens when you lose them. I'm asking about the specifics. My question is: if my character takes 30 damage, do they lose 30 endurance and 30 health? Or is it split somehow? That's what confuses me.

Damage (your '30') affects Endurance first and then the same amount is applied to Health. A class dependent multiplier is applied to Endurance value to get the Health value.

I see. That's about what I figured, then. Just to be clear, though: by "health value" do you mean that different classes will take a different amount of health damage for the same amount of endurance damage? Or just that they'll have a different health pool for the same endurance level?

It's the health pool, as I understand it.

That one is because you're using regeneration or healing effects during combat : Endurance is restored so you stay in combat longer, but health isn't restored and thus continues to go down. So you get the impression Health was more damaged, when in reality both went down at the same rate but health also went up due to spells.

That's something I noticed in my cleric-heavy group : The big healing auras meant my fighters could stay in combat for a long time, but I had to be careful since they could end up dying (or at least needing much more frequent rest) because of it

Lets use a little of maths here.
You have a character with Endurance (E) of 100 and Health (H) of 300 (or health pool as Hickory refers it).
In a battle, you lost 50 points of Endurance, this subtract also 50 points from the Health pool.
In a second battle, you lost E 30, then heal in middle of the battle and lost another E 20. This subtracted another H 50 points (current total of Health points: 200)

In a third battle, all your endurance was lost (not a single healing spell was used) but your character didn't die. Health is currently at 100 points, same than endurance. From this point, things are dangerous for your character.

There is another battle, you lost 10 points of endurance, now your Health pool is at 90. For the next battle, your endurance will be only 90 because it can't be more than your health. If Health reach 0, your character is begone of this realm.

Healing endurance is a easy thing to do. Recovering Health points, is another history.

Yeah, and what you described can happen in a single battle : There are some pretty powerful "healing aura" spells in the cleric's abilities that can maintain your frontliners' endurance at top level without you doing anything to actively heal your fighters, so you don't notice the health bar steadily diving, until you only have 50 health left and are all "Huh? how did I burn 250 health? My endurance never went under 75?" Happened to me once or twice, and it's a nasty little surprise when an apparently easy fight suddenly becomes desperate..