Take Hypnotic Specter for example:

![alt text][2]

When it attacks without being blocked in a two-headed giant game, do both opponents discard a card?

  [1]: http://www.wizards.com/magic/autocard.asp?name=Hypnotic%20specter
  [2]: http://mi.wizards.com/global/images/magic/general/hypnotic_specter.jpg