Touching the ball twice suggests he kicked it, there was separation and then he kicked it again. That didn't happen.
The decision VAR made was that he slipped into the ball with one foot while kicking it with the other. That he kicked the ball at the same time with both feet because there certainly wasn't any separation between the "first touch" and the "second"
Firstly, I honestly don't believe the footage is clear enough to categorically state he touched the ball with both feet. Anyone who says they can see that conclusively is a liar.
Secondly, I'm not sure there is anything in the rule that says you can't kick a ball with both feet at the same time:
Even if you're claiming there were milliseconds between one foot touching and then the other, without separation of ball and foot what difference does that make? There are milliseconds between foot and ball contact and separation on every shot. Unless you can prove they didn't both make contact at the same time, how can you disallow the goal?
It's honestly a scandalous decision and Atletico should be appealing to whatever authority there is available to them.
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Also, and this is an aside, does the wording of this law mean that if player takes a penalty during regular play and hits the post or bar, he cannot then score from the rebound unless it has touched the keeper or another player first?
https://www.theifab.com/laws/latest/the-penalty-kick/#offences-and-sanctions