The left hand gets a bit tricky. Instead of parenting the sword to the left hand, you parent the left hand to the sword. Now, if you move the sword, the left hand will move with it (because it's parented to the sword), but the right hand won't move (it isn't parented to the sword). You might say that it's a case of the right hand not knowing what the left is doing. However, when you move the right hand, the sword will move along properly (because it's parented to the right hand). And the left hand (parented to the sword) will move also, as intended. Confused yet? Good.