In another thread on repositioning, which I think has been exhausted, Wonderstell posited a theoretical example of a player getting two AoO at once, using both Upsetting Strike and Stylish Riposte. I certainly agree that this would grant two AoOs, so the exact text of the abilities isn't important for this question, the question is, whether they would both resolve at once or resolve sequentially.
His conclusion
My opinion is that the character would be able to take both AoOs, and that the second attack would benefit from any conditions my first attack applied. So if I tripped the enemy with my first attack, the enemy would suffer a -4 penalty to AC against my second attack.
I say this because if I'm able to take both AoOs, then it is impossible for them to occur at the same time. So we have to decide in which order to make these attacks, and they are considered to have happened in a chronological order since that's how time usually works.
I don't think the example was really relevant to the other thread anyway, and I'm not interested in arguing with him in particular about it (although he is certainly free to chime in here) but it did present an interesting situation and something I had to think about.
Initially I was in agreement with him. There are after all other times where two things happen at the same time, and generally the controller of the effect gets to determine the order, as is most beneficial to them. That principle would seem to apply.
Then I thought more about AoOs and some specific things that happen with them. One thing about an AoO is that, in general anyway, they are resolved before the action that triggers them is completed, for example, if I move away from you you get an attack at me at the position that I started from. In particular, this principle is why the trip-lock doesn't work, although you get an AoO for me standing up, you can't use it to trip me because your AoO occurs against me when I am prone.
Additionally, if I provoke against two opponents, and one goes for a trip and the other a regular attack, they are both resolved against me as I am at that point, the regular attacker doesn't have the option of waiting for the trip before making his attack. If two AoOs from two sources have to both be resolved simultaneously, then why would two AoOs from one source be any different?
This basically leaves the 'doesn't make sense' objection he had, that it is impossible to make two attacks at the same time. I'll grant you that assuming it is one weapon and they were regular attacks the weapon would have to hit once and then again (although I could certainly imagine a trip and a hit occurring at the same time.) I think the better way to pitcher this though is that these two attacks happen so fast (two AoOs in a response to one failed attack would be extraordinary fast after all, a result of some very specialized training in represented by the feats) that although they don't occur purely simultaneously, they are so quick that both are completed before the effects of either are manifested, sort of like the cinematic trope where the swordsman makes 2 cuts and then the victim falls in three pieces.) In game terms, both attacks would be made before either is resolved.
Anyway, I though it was an interesting hypothetical and I would be interested in others thoughts on it.
Obviously if your AoO brought them down, the other Player probably wouldn't have any need to roll it out.
I am not 100% sure if a player can abandon an AoO (thus get back the ability to take an AoO that round) as a result of that. Of course it is very rare when it would matter, first you would need the two AoOs and then one bringing the opponent down and then be in a situation where anyone would care if the second character had an AoO in that round or not, but it is another interesting question.
I would almost certainly agree with you as far as having the players act in order of initiative for simplicity, but that doesn't necessarily mean that the characters are not acting at the same time. As a practical matter we have to do one thing at a time, but I don't think I would apply the effects or tell the results until they had all been resolved. You do 10 damage, he does 5 damage, so I subtract 10 and 5 from the enemy HP and then see where they are and relay information to the players as to the result.
As a rule matter, I don't think there is anything that would say the initiative roll applies at all to taking of attacks of opportunity.
I think you might have made a mistake in the title.
Should be "Simultaneous Attack of Opportunities" or "Simultaneous Attacks", right?
That aside, you've explained my stance perfectly so I don't think I'll need to add anything.
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I can see three different conclusions to the "Simultaneous Attack of Opportunities" problem, which all stem from the fact that making two (opposing) actions at the same time isn't possible.
1) You'd have to choose between the two AoOs since taking both is impossible.
2) You'll get both, and have to decide in which order to take them for mechanical purposes.
3) You'll get both, and have to decide in which order to take them, but they are considered to have happened at the same time for mechanical purposes.
I think you might have made a mistake in the title.
Should be "Simultaneous Attack of Opportunities" or "Simultaneous Attacks", right?
You are correct, the brain went on autopilot and typed the wrong thing.