this is taken from reddit on the topic.
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despite what the others said, i want to add that in 5e, dual wielding alone, without others damage-adding abilities / spells (like hunters mark, hex, sneak attack), two-weapon fighting never beats two-handed fighting. i also want to add that currently, the offhand attack doesnt work as intended, because your DEX gets added to the damage roll instead to the attack roll. In 5e, you get your DEX added to the to-hit chance instead of the damage done.
lets look at the math (very long post, sorry for that. TL;DR at the bottom):
lv 1-3, without any feats and fighting styles, with a +3 in the attack modifier:
two-weapon fighting: 1d6+3 + 1d6 = 2d6+3 = 10
two-handed fighting: 2d6+3 = 10
so, exactly the same, but with two-handed fighting you can use your bonus action!
lets look at the fighting styles. two-weapon fighting style lets you add your attack modifier (+3) to damage for offhand attacks. great weapon fighting lets your reroll 1 or 2, which averages in +1 damage.
so with fighting styles:
two-weapon fighting: 1d6+3 + 1d6+3 = 2d6+6 = 13
two-handed fighting: 2d6+3 +1 = 11
so, with this setup you have more damage, but still cant use your bonus action. instead of great weapon fighting, which is not that good, you could also take defense to up your AC by 1.
no, lets look at level 5, first without feats. fighter gets extra attack, so you attack two times with you action.
two-weapon fighting: 1d6+3 + 1d6+3 + 1d6 = 3d6+6 = 16,5
two-handed fighting: 2d6+3 + 2d6+3 = 4d6+6 = 20
two-weapon fighting /w fighting style: 1d6+3 + 1d6+3 + 1d6+3 = 3d6+9 = 19,5
two-handed fighting /w fighting style: 2d6+3 + 2d6+3 +2 = 22
lets add feats (lv 4). two-weapon fighter takes either dual wielder or +1 in the attack modifier ability (STR or DEX). two-handed fighter takes GWM or PAM. Lets look at those feats:
dual wielder adds +1 damage on bcos you can wield non-light one-handed weapons (1d8 instead of 1d6) and +1 AC. if we assume that our two-weapon fighter uses DEX to attack, raising dex to +4 does the same, but also gives us better DEX saving throws and skill checks. so we raise DEX.
GWM gives us the ability to get a -5 to attack rolls and a +10 to damage rolls. it also gives the ability to attack again as a bonus action if we kill an enemy or criticaly hit. For the first part, lets just ignore the -5/+10 part.
PAM lets ur attack again as a bonus action no matter what and lets us take another attack of opportunity if someone enters our range (10ft).
now the damage at lv 5 with feats and fighting styles. lets take the defense fighting style (+1AC) for the two-handed fighter, so the AC stays the same:
two-weapon fighting: 1d6+4 + 1d6+4 + 1d6+4 = 3d6+12 = 22,5
two-handed fighting with PAM: 2d6+3 + 2d6+3 + 1d4+3 = 4d6 + 1d4 + 9 = 25,5
two-handed fighting with GWM, lets say you get a bonus action attack every other round: 2d6+3 + 2d6+3 + 1d6+1,5 = 5d6 + 7,5 = 25
what do we have now? two weapon fighting has +1 more DEX, so better DEX saving throws and better DEX ability checks. he also has ~3 less average damage per round. what does the two-handed fighter has? ~3 more average damage, the ability to either take opportunity attacks when someone enters the range or to deal more damage on low AC targets (which also increases average damage), and you possibly have your bonus action free for other stuff.
this discrepancy gets bigger and bigger the higher level you are. lets look at lvl 6 for example. the two-weapon fighter gets +1 damage on all attacks, so his damage increases to ~25,5. the two-handed fighter has both GWM and PAM, getting opportunity attacks when someone enters his reach (10ft), gets a save bonus action attack (1d4) where he can add his +10 damage from GWM, and potentially gets a 2d6 bonus action attack. also, if you take battlemaster, you have precision attack, which increases your effectiveness with GWM -5/+10. thus you have also way higher burst / nova damage. Also, action surge is better with two-handed weapons, because you only get an additional action, not an additional bonus action.
lets also look at GWM -5/+10 now. lets look at the damage when you attack wiht -5/+10 every attack at level 6: 2d6+3+10 + 2d6+3+10 + 1d4+3+10 = 4d6 + 1d4 + 9 + 30 = 14 + 2,5 + 9 + 30 = 55,5. Now you can say: but every attack has -5 and hits less likely. yes, but lets look at the damage of every attack: 2d6+3+10 = 20 damage on average. the two-weapon fighter has 25,5 damage with all three attacks on average, so you only need to hit 1,25 attacks every round, which means one attack per round, and two attack every 4th round, to deal equal damage than the two-weapon fighter, assuming the two-weapon fighter never misses. with buffs (bless, ..), advantage and possibly precision attack (if it gets added to BG3), you easily achieve this.
side note: GWM is regarded as a Tier S feat, because it is so strong. the -5/+10 increases your average damage more than you think, and it gets boosted even more with bonus action attacks, advantage and modifiers that gets added to your attack roll / tohit chance, like precision attack, bless, ..
just on top, the average and max damage you can deal if all attacks hit at level 6:
two-weapon fighting: 1d6+5 + 1d6+5 + 1d6+5 = 11 + 11 + 11 = 33
two-handed fighting: 2d6+3 + 2d6+3 + 2d6+3 = 15 + 15 +15 = 45
two-handed fighting /w GWM: 2d6+3+10 + 2d6+3+10 + 2d6+3+10 = 25 * 3 = 75, although you might need to use some resources like precision attack to hit all attacks with -5 to hit.
two-weapon fighting with action surge: (1d6+5) * 5 = 11 * 5 = 55
two-handed fighting /w GWM: (2d6+3+10) * 5 = 25 * 5 = 150
now, at lv 8 and 10, you cant raise your DEX any more, because it is already at 20 / +5. but you can raise your STR from +3 to +5, giving you even more damage.
TL;DR: two-weapon fighting is nearly always weaker on classes with extra attack and uses your bonus action, which limits other aspects like mobility. fighter espacially are so so much better two-handing than dual wielding. the only reason to dual wield (damage wise), is for rogues to proc sneak attack, or for some build with hunters mark / hex / .., which adds damage to every damage you deal. other than that, you are better of just two-handing. but dual wielding is not that bad, and if you like it, go for it. it is still effective.
