They can't. 5e specifically states that Sneak attack can only occur once per turn, that is per turn not per round. If the Rogue would react with advantage they could also apply Sneak attack, but thats the only way to use it twice in one round.
I think he meant qualify for sneak twice in a round, like multiclass im fighter 5 one attack you chose to not togle and the extra attack you do, rogue multiclass is something I play a lot with and that's something I very much intended to use.