Collectible Card Game Headquarters

The word "instead" means that it is a replacement effect, and the lack
of the words "When", "Whenever", or "At" mean that it is not a
triggered ability.

So, if we have a spell like "Shock deals two damage to target creature
or player" trying to resolve, and two Sanctuaries are in play, there
are two things trying to replace the "damage would be dealt to a
player" event. The affected player chooses which one to apply
first. (This choice is not really relevant here, since both effects do
the same thing.) This transforms the event into "Targetted player
removes the top 2 cards of his or her library from the game.". Then,
we look to see if there are any replacement abilities trying to
replace the modified event. There aren't any (the other Sanctuary
doesn't apply anymore), so we're done figuring out what's going to
happen.

Therefore, each point of damage only removes one card.

Note that with some replacement effects, like Gratuitous Violence,
multiples do have a bigger result, because the replaced event *can* be
replaced by another of the same ability. (The higher damage amount is
still *damage*, and thus another replacement effect looking for damage
will see it.) But that isn't the case here, since removing cards from
your library isn't damage anymore.