In this thread it was answered that since combat damage is simultaneous you either replace the entire event with one channel or the entire event with the other, even if one is blocked and the other isn't. This leads to two follow-up questions I have that make more sense with the other answer.

First, if you're playing two-headed giant and the two creatures are dealing damage to different heads (or are blocked by creatures controlled by different heads), does this change the answer? It's a single event, but the affected object/player who is making the choice is different for each of the damage sources.

Secondly, if you have a Protean Hydra enchanted by a Pariah which blocks one of the two attacking creatures (no channel harms this time), now you have a single event with two applicable replacement effects (hydra and pariah). Lets say you choose to apply hydra's effect first. Now when you apply pariah's effect, you now have damage that would be dealt to hydra again. However, you've already applied hydra's replacement effect so this time the damage doesn't get prevented, instead it gets marked on the hydra? If they were treated as different original events then you could apply hydra's effect to each separately.

Lastly, do either of those scenarios shed more light on the original?

Thanks,
Matt

Channel Harm only prevents damage that would be dealt to you and permanents you control; it makes no attempt to prevent damage that would be dealt to your teammate or your teammate's permanents. There's no overlap between those two sets, so there's no way for the effect from Channel Harm to have multiple “affected players” that need to make choices.

As for Pariah and Protean Hydra, the Hydra has a prevention effect, not a replacement effect–the two are similar, but they have at least one very big difference: unless the damage is unpreventable, prevention effects can and will invoke themselves repeatedly to the same event if they're still applicable. So when in your example you apply the Hydra's effect first, then Pariah's, the Hydra's prevention effect becomes applicable to the event once more, and is applied a second time to prevent that damage as well.

So, since both questions are moot, neither of them really sheds any light on anything.

It's probably worth noting that while the “all or nothing” situation created by multiple Channel Harms may be somewhat weird at first glance, it's already been around for years thanks to cards like Empyrial Archangel and Palisade Giant without causing any significant amount of confusion or grief in casual play, nor any at all in competitive play. It's at best a corner case, and there's little benefit to be gained from hashing it out repeatedly here in Rules Q&A. If anyone would like to continue discussing it, I suggest taking the matter to Rules Theory and Templating on the Wizards forums.