I know all enemies move at same step, but, when 2 monsters would enter at the same room with a trap... who activate the trap and receive the damage?
Wesley is correct, the players decide. This is also true with barriers. It is also true if the enemies have different speeds. For example a monster and animal move onto a trap; the players may decide to assign the trap to either, if they choose the monster, the animal will move through the tile if it has movement remaining.