I actually saw people play out of sequence - some buskers or homeless or whatever people - just used to playing so many games against each other they'd get through the openings quickly by each person making several moves. It seemed too fast for me to see that they kept a correct pace, but maybe. I believe the games were very rapid. they stopped and started switching turns normally as soon as they were out of the opening. (so this refers to just a handful of moves at most.)
so with a bit of sportsmanship it's possible to actually play that way over the board with somebody who's cooperating with you to get through the opening quickly.
"In order to save time, and to prevent useless exchanges, it was agreed that the first player should make his (let us say) fifteen moves all at once, without, however, crossing the middle line of the board; after which the adversary was entitled to play up at once an equal number of counter moves... these preliminary maneuvers the Arabs called Ta'biyat, which signifies 'the drawing up of troops in battle array'."
(quoted from Duncan Forbes "Observations on the Origin and Progress of Chess", 1855 - I first came across the variant in Edward Lasker's "The Adventure of Chess".)
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Cheers!