Again, I would start from the history. What were 2-handers used for? That will tell you how to model them in the game.

The basic Medieval two-handed weapon was a kind of polearm - the halberd, bill, poleaxe etc. These had a number of advantages compared to swords:

(1) better at penetrating armour (or concussing the armoured opponent)

(2) more reach, so functions almost like a spear against cavalry.

There are some differences between these 2-H weapons - the halberd and poleaxe are heavier and more AP than the bill; the halberd has more reach than the poleaxe etc. But these are nuances.

The primary disadvantage was that you don't get to use a shield, at least in melee (you could sling one on your back, I guess, and use it versus missiles like the MTW Varangians), although this was less of an issue when you have plate armour to fall back on.

I think MTW did a fair job of modelling these kind of weapons: higher base attack stat, AP, +3 attack/+1 defence vs horses, and (for non-two handers) a shield bonus that disappeared as the base armour got more advanced.

I think they should start to replace sword and shield men towards the middle of the game. The S&S should become obsolete. Early two handers - like halberd militia - should suffer a little due to their lack of armour. I'd see them as "flankers" or cav killers (not cav stoppers). But the later two handers, like DEK, should basically replace the armoured swordsman type as they did historically.

Having dealt with pole-arms and the standard S&S unit, there are a few more "novelty" units to consider - specifically, the two-handed sword (Zweihander etc) and the sword/buckler men.

The 2-H sword probably should be AP, just to reflect is weight and bludgeoning power, but unlike a polearm has no particular advantage against cavalry. It should have a high attack, but low defence - making it the archetypal flanker sort of unit. Again, I think MTW did a good job modelling this. It should be lethal against the flanks of pikes etc, but I would not see it necessarily besting comparable S&S units. I'd see that more as a fair match-up: high offence vs high defence. The 2-H sword was a bit of a rarity, mainly introduced as a novelty to counter pikes.

The sword/buckler were another counter-pike novelty. I don't see them doing well against plate armoured men-at-arms (no AP). Historically, they were not a sure kill against pikes either. I'd probably let the outcome be decided by troop quality. I suspect the best pikemen (the Swiss) beat sword and buckler men more often than they lost, but I'll leave that to the historians to confirm or refute.

But 2-H swords and sword/buckler were much more niche weapons than polearms, which had become the man-at-arms weapon of choice for fighting on foot by the end of the period.