I took this statement:
'The caretaker tells you each, individually, that together you can see either 18 or 20 statues. '
as being:
size(see_A) + size(see_B) = 18 or 20
as opposed to: size(see_A union see_B) = 18 or 20
i would have assumed they would add the word distinct if they wanted to not double count overlapping statues.