Huh, I assumed since you independently determined the difficulty pools, the attacks could independently target. I'm forced to concede that your interpretation is consistent with a strict reading of the rules and is strongly implied by the existence of the Spitfire talent.
You always use the higher difficulty and increase it by one.
Clarification: this is how autofire works, but (as Dono outlined) not attacking with two weapons.
Firing at two targets simultaneously is improbably difficult, and even in settings where it is appropriate (IMO, Star Wars joins The Matrix and western gunslinger flics here), it requires some level of bad-assery or specialized training. Hence, the Spitfire talent!
The implementation of Spitfire is pretty bizarre though. If you shoot a mook at close range, you get to hit a nemesis with a personal deflector field at range? It gets even worse if you have light repeating blasters, or something else with Autofire. Or does combined check mean something else?
maybe you must say before attack, that you want to shoot two target, then take bigger difficulty.