I would add to the first answer that they act in a similar way when they find the conch and first start messing around to see if they can make any sounds with it.

Ralph, at this point, seems pretty brainless.  Piggy is providing the brain and Ralph, the physical ability.  Ralph thinks the conch is a rock, but Piggy knows what it is.  Ralph has no idea what to do with it, Piggy tells him.  Piggy knows how the conch is supposed to be blown, but is apparently unable or unwilling to try to do it himself.  So when you look at this part of the chapter, you can see Piggy acting as the brain and Ralph carrying out his orders.

One of the things that many people have decided about the book, is that Ralph stands for civilization in a way, but Piggy stands for reason and science.  If you follow the way that they take the conch and then use it to act as the symbol of authority, allowing whomever has it to speak, Ralph acts as the mouthpiece in some ways for Piggy's ideas.

Piggy's weight and general appearance have always made it certain that he will not be considered the leader of anything, and they cannot look to him, so Ralph in some sense acts as the front man, but the ideas were from Piggy, so they are, in effect, acting as one person.