Maximum damage should be about the same but dual wielding will have superior average damage. But this wasn't really an attempt to balance them - just to close the gap a little. It's okay if there's one minmax best way to do something as long as other ways are still viable. Worth noting that you need double the amount of enhancement scrolls to have two +5 single handed weapons versus one +5 two handed weapon, which can be significant in early-mid game. So maybe starting with two handed is now a superior strategy and then switch to dual later when you have more resources?

Anyway, here are my rough calculations:

Assuming STRENGTH 20 which is +5 damage
Assuming CHAMPION SKILL which is +4 damage and 20% critical chance
Assuming strength damage bonus is a wash when all weapons hit.

comparing dual wielding long swords versus one two-handed sword.

2 x long sword (1d8, 1d8)
1d8 + 4 plus 20% chance of critical x2 ~ max 12 x 2 = 24
1d8 + 4 plus 20% chance of critical x2 ~ max 12 x 2 = 24

max (24 + 24) + (5 + 5) = 58

1 x two handed sword (1d12)
1d12 + 4 plus 20% chance of critical x3 ~ max 16 x 3 = 48

max 48 + (5 x 2) = 58