At no point do the setup instructions say to use two ettins. They simply instruct you to place the ettins (however many there are) on the northern-most tile and choose one (however many there are) to be Mauler.
Maybe it's how the person reads it. The first time I did a 2p First Blood I remember seeing something similar and thinking "so should I have two?". But yes, always respect the group limits on the cards unless something in the quest explicitly says to ignore group limits.
Good luck questing!