How are the twins shown to be so alike throughout chapter six?

Sam and Eric not only look alike, they share each other's personality traits. Golding really has them operate as one person, "The twins shared their identical laughter...." When they see, what they perceive to be, the beast, Golding has them act in unison,

They became motionless, gripped in each other’s arms, four unwinking eyes aimed and two mouths open. Far beneath them..."