The Heavy Blade Opportunity feat (PHB) lets a character use any of their at-will powers for their opportunity attacks.
Opportunity attacks, on the other hand, target the triggering creature.
What happens when using an at-will power that mandates to target two different attacks, such as the tempest-fighter power Dual Strike (MP)? Is the second attack wasted because you can only target the triggerer? Or does it let you target a second, non-triggering enemy?