Gameplay » LEGEND Pokemon | Gameplay » Starting the Game
What if during setup the only Pokemon cards in my hand are Pokemon LEGEND halves?
Since you have no Basic Pokemon to place Active you will let your opponent look at your hand, then shuffle and redraw your hand as described in the rulebook.
Source: HeartGold & SoulSilver FAQ (2010-02-11), PUI Rules Team (2010-02-11)