Would love official stance on this fella... I'm inclined to think it is two rolls. one for each target, individually accounting for each target's effects (so hitting the unit at the back, if it is adjacent to a Defender, could get intercepted, etc.).
Mostly because it is innate. I feel like if the intent was a single roll hitting two targets, I'd have made that a green Ability.
I'd say it is one roll and do the damage to both. But you could do two rolls if you wanted. I don't think it would make a huge difference as long as you are consistent in how you do it. Maybe Adam or Josh will reply.