And in particular, when you play with 2 players do you deal out all the cards the from the deck? Or is there some undealt part that players draw from during the game? I mean, if you deal them all then you'll just know what's in your opponent's hand?
This is going to sound trivial, but actually it's part of this theory/hypothesis I have for this anime/manga series The Quintessential Quintuplets, where I think the game Sevens(/Fan Tan/Domino) is symbolic of something in the series (namely something to do with the number 7). My thought is that the card game Sevens (and thus the number 7) is so important that it even leads to a mistake like this. So if this were a mistake, then it's possible that Sevens is really important. Or something.
- Actually there have been previous discussions of the game Sevens in this series. See: The importance of "Sevens"
From S02E12 of The Quintessential Quintuplets
The 2 kids pictured above, a young Nakano and the young Fuutarou, are playing
in the English dub version: 'Sevens'. (This is also in the English subtitles for the Japanese version.)
in the original Japanese version: 'Shichi Narabe (7並べ)', which is what Wikipedia calls a(/the?) Japanese variation of Sevens. In the upper right part of the Wikipedia page, you can see that it says 3-8 players, so I guess that includes the Japanese Sevens.
You can even see how the cards are laid out that they really are playing (some variation of) Sevens.
I've read only a bit of the manga, but I'm pretty sure no other kids were playing with the 2.
Additionally, Sevens has been played earlier in the series (S01E09 where it's still Shichi Narabe [7並べ], but it's translated as 'Fan Tan' instead of 'Sevens', again by both the English dub and the subtitles for the original Japanese), but there were at least 3 players then.