How do multiple Cruciatus Curses affect someone?

Question

So what happens if someone is hit by two or more Cruciatus curses? From different people? I was under the impression that the curse was designed to inflict a lot of pain - obviously enough to break people (the Longbottoms). But that was just from being continuously tortured by Bellatrix, right?

So what happens if you have two people using the curse on one poor sod? Will the pain increase, or are they already in the maximum amount of pain they can be in? Will it just kill them? If you had two people throwing the curse, and one hated the victim more than the other, would their curse be more potent?

Yikes, sorry. Not sure if I'm allowed to ask so many questions all in one.

Answer 1

The effects may be enhanced

There's good reason to think that spells can "stack" for greater effect. For example, normally a Stunning Spell simply knocks someone out. But when Professor McGonagall was hit with several Stunning spells at once, the effect was far more dramatic:

'Leave him alone! Alone, I say!' said Professor McGonagall's voice through the darkness. 'On what grounds are you attacking him? He has done nothing, nothing to warrant such -'

Hermione, Parvati and Lavender all screamed. The figures around the cabin had shot no fewer than four Stunners at Professor McGonagall. Halfway between cabin and castle the red beams collided with her; for a moment she looked luminous and glowed an eerie red, then she lifted right off her feet, landed hard on her back, and moved no more.

—Harry Potter and the Order of the Phoenix

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'It'll be his giant blood,' said Hermione shakily. 'Its very hard to Stun a giant, they're like trolls, really tough... but poor Professor McGonagall... four Stunners straight in the chest and she's not exactly young, is she?'

—Harry Potter and the Order of the Phoenix

Normally the Stunning Spell can be countered with the simple intonation of Renervate, but in McGonagall's case, the effects were appreciably longer-lasting:

'She's not here, Potter,' said Madam Pomfrey sadly. 'She was transferred to St Mungo's this morning. Four Stunning Spells straight to the chest at her age? It's a wonder they didn't kill her.'

—Harry Potter and the Order of the Phoenix

Similarly, when Harry, Ron, and Hermione all attempted to disarm Snape in Prisoner of Azkaban, the effect of the combined spell was significantly more potent:

"Expelliarmus!" he yelled -- except that his wasn't the only voice that shouted. There was a blast that made the door rattle on its hinges; Snape was lifted off his feet and slammed into the wall, then slid down it to the floor, a trickle of blood oozing from under his hair. He had been knocked out.

Harry looked around. Both Ron and Hermione had tried to disarm Snape at exactly the same moment. Snape's wand soared in a high arc and landed on the bed next to Crookshanks.

—Harry Potter and the Prisoner of Azkaban

Generally speaking, it seems multiple instances of the same spell cast simultaneously result in more physical damage. In the case of Expelliarmus, the physical damage appears to be a side effect of its intended use (a consequence of the spell having "mass," if you will), whereas in the case of the Stunning Spell, it seems to be a combination of stacked main effects and stacked raw power.

Of course, the Cruciatus Curse is significantly more advanced and dangerous magic, but I suspect the same principle applies.

We're not told whether the Cruciatus Curse causes the maximum level of pain that a human can experience, but if it does not, there's good cause to believe that multiple such curses will have enhanced effect. Even if it does, it is possible that multiple Cruciatus Curses will have a physical effect on someone due to their raw power, as with Expelliarmus.

Answer 2

I assume it depends on the relative magical power of the person casting it and how much effort they're putting into hurting their victim. For example, when Voldemort cast it, it really seems as if Harry is in as much pain as can possibly be felt;

"Voldemort raised his wand, and before Harry could do anything to defend himself, before he could even move, he had been hit again by the Cruciatus curse. The pain was so intense, so all-consuming, that he no longer knew where he was... white-hot knives were piercing every inch of his skin, his head was surely going to burst with pain; he was screaming more loudly than he'd ever screamed in his life — "

_{^{Harry Potter and the Goblet of Fire}}

Admittedly, you could tack on "And then it hurt even more" but beyond the point described, anything else would simply be an afterthought.

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By comparison, when Harry himself cast it, the pain was relatively limited and there seems ample scope for multiple users to get in on the action.

Hatred rose in Harry such as he had never known before; he flung himself out from behind the fountain and bellowed, ‘Crucio!’
Bellatrix screamed: the spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had – she was already back on her feet, breathless, no longer laughing.

_{^{Harry Potter and the Order of the Phoenix}}