This is addressed in the starter rulebook on page 10:
If an attack affects more than one defender, make a seperate attack roll and defense roll for each defender.
But in the comprehensive rulebook in section 601.1a it says:
If there are multiple targets, only one attack roll is made and applied to each target.
As these seem to be contradictory, it would be nice to get an official clarification. This has probably been addressed on the official forums and if I can find an answer, I will post it. In the meantime, I would assume the comprehensive rulebook takes precedence over the starter rulebook.