Hello!

I'm wondering about the specifics about the wording "This attack may be used 1 additional time" on the Super Shotgun (Double Barreled Blast) and Gatling Gun (Endless Fire).

Does this attack have to be on the same target or can it be used on several targets? I can't find the exact wording in the rulebook.

i generally think it's fair to assume that if it doesn't specify that the attack must be the same target then you would be free to use it on a secondary target

You can target something else unless specified otherwise. There's an invader card that specifies it's second attack must target a different marine from the first.