If they were in the same cabin with the same muster station as they had on the first leg of the cruise, I can see where it is silly to have them go through it again. How hard is it to be able to flag the muster station roster to identify the B2B passengers? I'm sure someone will say 'how hard is it to just show up again?' How far do the lines want to carry this? If a ship is doing a 100 day world cruise with 20 different segments, does this mean those aboard for the full 100 days need to attend 20 muster drills?
I attend the drills and when I do my B2B on the Liberty Of The Seas this coming December, I will go to both drills if required because I don't want to tick off the Head Mountie and get booted.
If the cruise lines are truly serious about it, they'd not sell any alcohol until after embarkation and the drill were complete. Just a couple of drinks would make some people not remember much. But they won't go that far because in the end it hits the bottom line. For that matter, I'd wager most passengers even if sober don't pay much attention.