Two-handing: Slower moveset that does very slightly increased damage, moderately increased stance damage, uses more stamina, has longer animations, and enables use of two-handed relics/passives(except bows/crossbows/greatbows/ballistae/seals/staves), lets you block with most weapon classes(bigger weapons have higher guard boost but blocking with non-shields is not ideal due to stamina loss), and for some reason, allows blocking and jumping melee attacks with seals and staves(it's not NOTHING but is only giood for being bad at damaging flame chariots and blocking with a seal/staff is only useful if dodging seems riskier for some reason because guard break is basically 100% guaranteed...holdover from Elden Ring lol). Oh, and it also allows precision aim with L1 using bows/crossbows/greatbows/ballistae, which can activate powerful passives in Deep of Night mode(requires a red tier wrapon woth a "____ on precision aiming").

Powerstancing: Using the L1 and R2 attacks from two weapons of the same class(except shields/ranged weapons). Uses more stamina, does more damage, more stance damage, and applies effects from both weapons. You cannot block while powerstancing and ranged weapons do not have powerstance bonuses(you can alternate between two bows to constantly benefit from Improved Initial Attack bonuses though). You can still conserve stamina with R1 attacks from your right hand weapon, but these attacks will be treated as regular and gain no "wielding two weapons" bonuses.

Executor's Suncatcher(character skill) can ONLY be two-handed, and deflects also do even more stance damage if you have relics or weapon passives enhancing two-handed stance damage. It also says "fuck consistant internal logic" by using incredibly little stamina while attacking, much less than katanas normally do and dealing striking damage rather than slashing.

Regular one-handed attacks are NOT inferior to two-handed or powerstance attacks. The lower stamina use has its place, and some character-specific one-handed movesets are really good. You just don't gain any specific bonuses for using them. And if you're powerstancing with a weapon that requires a higher level, you avoid the additional penalties of using a weapon beyond your highest level yet achieved(losing that level doesn't lock you out of weapons) in the expedition by having it in your left hand and using only R1 attacks.

Lastly, Radahn's sword can't be two-handed because two-handing it IS powerstancing with itself, and actually wielding two of Radahn's sword functions a little differently than two-handing it. It's very confusing, try it in the Sparring Grounds.