A player has build 2 decks from their sealed pool, and has sleeved them differently. Their opponent helped them build their decks, so knows the general strategy of each. Players are now shuffling for game 2, and ask who has to present first (potentially allowing sideboarding based on the decision which deck is being played).

I ruled that the player who decides who plays first (so the player who lost game 1) has to present first, based on the idea that they're most likely the first active player, but would love others thoughts on the question.

That seems like the most sensible solution for Regular, and was my first instinct too.

Another potential solution (if you think the players might enjoy it) is have the Player A secretly write down what deck they're going to play (e.g. Merfolk or Vampires), and let Player N decide what sideboarding (if any) they want to make. Then have them shuffle and present their decks. The issue with this option is that it relies on both players being somewhat honest - that Player A won't just pick up the other deck once the sideboarding decision has been made, and that Player N won't change their sideboarding once they see what deck is being picked up. But it can work if you know the two players will be good sports about the whole thing and enjoy the guessing game.

Sideboarding 40 cards in, shuffling, and then sideboarding 40 cards out is also a clean solution that will avoid any ambiguity or shenanigan, as long as the player can do that in a reasonable time (and if not, maybe he or she is focusing on the wrong part of the game to improve his or her win %).

- Emilien

Originally posted by Emilien Wild:

Sideboarding 40 cards in, shuffling, and then sideboarding 40 cards out is also a clean solution

The two decks are sleeved differently, as per OP.

Edited Norman Ralph (Jan. 15, 2018 05:47:21 AM)

“when are you done sideboarding, I will count one, two, three: when I say three, present your deck”.

Originally posted by Francesco Scialpi:

“when are you done sideboarding, I will count one, two, three: when I say three, present your deck”.

I'm not sure the counting down is necessary but I like the idea of this. Just have both players say when they're done boarding. Guy with 2 decks knows which hes going to present (and could always bluff board both if they want), and making single deck guy say he's done stops any shenanigans.

I would allow the player with 2 separate decks to present last in this scenario
In the hypothetical scenario where they both keep changing, at some point they need to continue playing, so I would force them to for example “present your deck 1 minute from now”. The player with 2 separate decks can shuffle both of them and have them present-ready, being able to instantaneously swap between the decks. The other player however, would have to look at his deck and shuffle again every time he makes a change. So with only a few seconds left, the other player's deck has to be ‘locked in’ while the 2-deck player can easily make the swap still.
Which is basically the same as telling the other player to pick first.

Now if they both have 2 separate 40-card decks however…

Edited Toby Hazes (Jan. 15, 2018 12:28:46 PM)

Originally posted by Norman Ralph:

The two decks are sleeved differently, as per OP.

That player took a lot of efforts to paint himself or herself in that corner.

That scenario requires a lot of specific things, many of them being in the player control: using different sleeve, the opponent knowing both of the decks content, the opponent having a different sideboard plan for each of them, the player knowing that…

That means that it's both easy to avoid, and that this specific scenario is of little educational value, and best left out for social gathering amongst people enjoying dissecting corner cases.

- Emilien