At it's core; Spitfire allows you to make a two-weapon attack at one target and if you succeed you can hit anyone else in range with your second shot; be they a minion Stormtrooper or Darth Vader. That's the big advantage of Spitfire; applying minion defenses to BBEGs for 2 Advantage.
I don’t believe that Autofire says that you have to hit the more difficult target first, but that was part of the developer clarification.
I’d apply the same clarification here. But maybe that’s just me.