“Don't try the paranormal until you know what's normal.” - Granny Weatherwax
If playing with just 2 Heros, do the Heros get the additional (non action) attack they would get in the vanilla edition
That's a good question that is currently not address by the FAQ.
I believe you should apply the new rule nonetheless also to Forgotten Souls, Nature's Ire and Dark Elements.
They get that in the vanilla edition?
Recently (since Heirs of Blood, I believe), in 2-Heroes "normal" campaign and Road to Legend campaign a new rule has been officially implemented. You can find this rule added to the FAQ and to the rulebook of the base game reprint.
In a 2-heroes game, every hero may perform a free "normal" attack beyond his 2 actions. If he couldn't or wouldn't perform that attack by the end of the turn, he recovers 2 damage.
I think the main purpose of this rule is backing up for the lack of a healer or the lack of a heavy hitter.