So far playing through the campaign for the first time with one opponent (controlling two heroes) I'm left with the impression that the game is horribly imbalanced for 2 heroes. I'm watching my opponent struggling to even come close to a win every time, I'm not feeling challenged in beating the heroes, and I can't see anything that they're consistently doing wrong (one or two little things I disagree with, but I don't think two players will always agree on the right tactical decision anyway so I don't think that's it).
The rate of reinforcement is the same for 2 heroes as it is with 4 heroes (obviously not an issue on no reinforcement stages). So if the heroes are trying to battle through a group of monsters and are knocking off (for example) 1 monster per hero per turn, with one reinforcement the 4 heroes are getting a net rate of 3 less monsters on the battlefield a turn while 2 heroes are getting a net rate of 1 less monster on the battlefield a turn.
Secondly monster groups are only slightly reduced for less heroes. A goblin group for 4 heroes is 4 white + 1 red, while for 2 heroes it's 2 white + 1 red. The red monsters are significantly tougher than white monsters, let's conservatively call them worth 1.5 goblins. So 4 heroes fight 5.5 goblins, 2 heroes fight 3.5 goblins, they only have 50% as many actions between them compared to 4 heroes to deal with the goblins but there are 64% of the number of goblins 4 heroes have to deal with. In other words 2 heroes have to do 28% more work than 4 heroes but get nothing to make up for that.
Thirdly though the enemy player starts with more cards for more heroes they still get one extra card per turn regardless of the number of heroes. It's not possible to quantify the impact that has as clearly as the first two points, but it's undeniable that getting the same amount of cards but less heroes to absorb the hits will disadvantage the heroes even more.
Has anyone else found this problem and discovered what 2 heroes need to be doing to make up for the disadvantage? Is there some balance adjustment for 2 heroes I've missed?
My current thoughts regarding house ruling this is to give the lesser numbered heroes a "motivation boost". That is, because there are less of them they are pished to do more because the odds are more dire. This manifests in them each getting an extra action (possibly just an extra move action rather than any type of action) per turn. With 3 heroes, which presumably has the same issue but perhaps not as extreme, the entire party gets one extra action which one hero may take per turn.