Question about etiquette and reroll (LVL2)

I would say it's the creators choice if he wants to ask the winner. But it may take a while for both of you to chat about it and make a decision considering there are some winners that doesn't login daily.
I think if the groups aren't ratio-based then it doesn't matter which one you reroll.
And for the second case where it's just for level don't think it matters either.

creator imo, but i suppose offering the chance to winner to decide is okay too. as far as which to choose, personally i'd give them the one with lower entries and re-roll the one with high entry count, just in case you end up re-rolling several times and run out of people to re-roll too. (yes, been there, it happens too)

Giveaway creator's choice, to be sure. They're the ones determining which groups to send to, and thus they determine the priority factor of those groups. The entrant already made their judgement call that the entry "cost" was worth it for both groups, so they've already had their say.
Not to mention, as mmm indicated, it could turn into an unnecessary hassle if it moved to being a winner-based decision, and it rarely matters to the winner besides. d3m4n's point is certainly one to consider as well- taking too long on getting a winner in low entry giveaways can potentially leave you without a valid winner. [I believe I also have had that happen before- at the least, I've certainly come very close to it.]

But, it's a moot point- if you DO want to switch it around, so long as you still haven't sent the second game or gotten the reroll on it, you can just have the winner mark the giveaway he prefers as being received, and then swap around the remaining key to go toward the other group. [Closing the existing reroll support ticket and opening a new one.]

So, since as Giveaway Creator it's your call, if your call is "winner's choice", then by all means, offer them that option.

it's not up to the winner. :3

This is kinda curiosity. If you don't find answer, try to don't make GAs which ends in the same time next time. If one ended just 1 second earlier, you would not have to solve this dilemma.

I think creator is the one who should decide.

Edit: also you should make the reroll as soon as possible so you cannot wait for a winner than they give you an answer.

If it happened to me as a winner, I would ask to reroll the giveaway with the lowest level requirement, or the one shared with the biggest groups, in order to give more people another chance at winning; as a giveaway creator, I would most likely follow the same reasoning.

It's up to the winner imho. If people win the same game from different givers, it is also their choice where they request a re-roll. That's my simple approach to that topic.