Opponent has out an {Ornithopter}. He has no land, put he has two {Mox Opal}s in hand. He wants to know if he can play them both and activate metalcraft, tap for mana before he has to sack one. Basically,
{Ornithopter}
{Mox Opal}
{Mox Opal}
Legendary trigger.
Tap both for mana.
Resolve trigger.
Does that work?
No, because the legend rule is a state-based action.
704.5k (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=704.5k): If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners' graveyards. This is called the "legend rule."
704.3. (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=704.3.): Whenever a player would get priority (see rule 116, "Timing and Priority"), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. This process also occurs during the cleanup step (see rule 514), except that if no state- based actions are performed as the result of the step's first check and no triggered abilities are waiting to be put on the stack, then no player gets priority and the step ends.