The trick is the combination of one player lifting it (or throwing it) to the other player's head and the second player heading it back. If you want to stick to the wording of the law and say the law says "a player" then I'd say the fact they are using more than one player to avoid getting caught is on its own circumventing the law.
In other words they know one player can't do it, so they use more than one player to circumvent it. How much more crafty (tricky) can you get.
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The first one who meant it as a trick. In the OP its clear they both did so i'd say the keeper. But TBH I won't be too upset if you deservedly caution them both because the both take part in circumventing the law and making a mockery out of it. But that wont help game control.
So two defenders take possession of the ball near to their own goal line and repeatedly pass the ball between themselves to waste time..........is this an offence? I think not.....This is clearly circumvention.... Are people seriously claiming it isn't? Warn once, then card. Some people are getting way, way too hung up on the word 'trick' and trying to find hidden meanings that aren't there.
In either scenario its up to the opposition to do something about it......
Get real people, stop creating problems for yourselves....