SevenSidedDie is right. I just want to amplify what he says. This started as a supporting comment but quickly grew beyond what would work in that context.
There is no implicit message that only one character can defend a given target. Very little in Dungeon World is implicit. In fact, some things, like the Hack and Slash requirement that you be attacking an opponent in melee, are so explicit that people sometimes are confused by them - assuming that Hack and Slash is just a To-Hit roll and not a Move, for example.
One explicit thing is that what works in the fiction is critical - more critical than SSD stated. The central precept of Dungeon World is always Fiction First. If the players can explain how they both defend someone / something, then they can. Otherwise, no. Dungeon World works like this:
Fiction->Move-v
^----------|
Players engage in the Fiction - the story being told at the table. They have a conversation where a situation is described and actions and their consequences are stated. The GM says, "You enter the tavern and the heat and light of the fire are both startling and welcome after the cold, moonless darkness of the road." A player says, "I hang my cloak on one of the hooks by the door and order hot wine for all of us." The GM says, "The tavern keeper nods at your order and you can see a table open, not as close to the fireplace nor quite as big as you might like. As you thread your way through to your table you catch a glimpse of a strange sigil carved into the hearthstone." All of this is the Fiction.
Eventually, something in that story triggers a Move - a piece of the rules that specify a series of choices or actions in terms of the game mechanics. Each move specifies a trigger. A player says, "Do I recognize the symbol carved into the hearth?" Now the GM says, "Maybe you should Spout Lore?" By consulting her accumulated knowledge about something, the player triggered a Move. The book specifies what you roll and what happens given the result. "I got an 8 - tell me something interesting!" The GM says, "You've seen that sign before, you are sure. You can't say for sure where, but something tells you it was on a tapestry last time, not carved in stone." This move is now resolved - the player rolled, the GM gave something interesting.
Now, the result of the move has to feed back into the fiction. It's up to everyone to do this, not just the GM, not just the player who rolled. "I quietly ask if anyone else noticed that sign on the hearth," says the rolling PC, "and I tell them about seeing it on a tapestry somewhere." Another player says, "Was it at the shrine we visited on our way out of Legerre? That place was rotten with tapestries." A third says, "No, those were all pastoral scenes! That sign was on the tapestry hanging behind the altar in Gruthak's tomb!" These players are contributing options to the fiction - they are having a conversation that implies a selection between choices - is this a friendly tavern or a haven for cultists? The result of that move must have an effect on the fiction one way or another.
The conversation will then continue until another move is triggered. Play proceeds in this fiction->move loop until the end of the session.
So it really does boil down to what's happening in the story - not the rules of the Defend or Aid or Hinder moves.
