The problem as stated seems ill posed to me. If Santa wakes up when
either quorum is ready, then it's never true that both are waiting
unless they arrive simultaneously. Either that's a zero probability
event that can be safely ignored, or it needs to be handled by
arbitration in some form. Attempting to resolve contention by giving
preference to one or the other shifts the problem to that of
determining whether they're close enough to be considered
simultaneous, with edge cases needing to be resolved by arbitration,
and so on.
I expected the problem to include the ability for additional elves to wait while an elf tour is taking place: thus would obviously then allow for complete parties of both elves and reindeer to be awaiting Santa's return simultaneously.
I'm not familiar with the original puzzle, but this is, at least, not the problem the author addressed.
Confused: they are 'waiting' if they are in the queue when the queue is polled. What else could 'waiting' mean? How is time involved at all?