It's one roll, but potentially two hits. You spend two advantage to hit with the secondary weapon.The "each additional hit" part seems to be the biggest point of confusion. "Each" implies there is potentially more than one additional hit, however with dual-wielding that isn't the case. Narratively, dual-wielding allows for two weapons to strike, but it's still one hit, mechanically. I feel that's an important distinction in the discussion of "hits", making it even murkier.
If we assume "hit" means "success", then the word "additional" becomes problematic, as in an attack all successes count. Perhaps it should be worded, "each success"? Even so, I'm not sure causing "free damage" based on successes alone is worth all that much. You'd need to roll 6 successes to even harm a stormtrooper, which is a rather substantial amount of successes to be rolled.
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I mentioned this. The confusion isn't about dual-wielding, it's about dual-wielding with Spitfire.