Assuming 65% chance to hit, see this math
Level 1 fighter with a greatsword does 2d6+3 x 0.65 = 6.5 damage per round
Level 2 fighter with Great Weapon Fighting does (8.33+3)*2 x 0.65 = 7.37 damage per round (reroll 1 and 2s)
Assuming Great Weapon Fighting for the rest
Level 5 fighter attacks 2x with greatsword for (8.33+4)*2 x 0.65 = 16 damage
Level 5 fighter using Great Weapon Master deals (8.33+3+10)*2 x 0.4 = 17 damage (minus 25% chance to hit because GWM)
Level 11 fighter attacks 3x with greatsword for (8.33+5)*3 x 0.65 = 26 damage
Level 11 fighter using GWM deals (8.33+4+10)*3 x 0.4 = 26.8 damage
Level 1 fighter wielding 2 shortswords does (1d6+3)+(1d6) x 0.65 = 6.5 damage
Level 2 fighter with Two-Weapon Fighting does (1d6+3)*2 x 0.65 = 8.45 damage
Assuming TWF for the rest
Level 5 fighter does (1d6+4)*3 x 0.65 = 14.62 damage
Level 5 fighter who chose Dual Wielder Feat does (1d8+3)*3 x 0.65 = 14.62 damage but also gets +1 AC
Level 11 fighter does (1d6+5)*4*0.65 = 22.1 damage (the one who chose Dual Wielder Feat does the same damage but also gets +1 AC)
In sum, at level 11, a dual-wielding fighter (with Two-Weapon Fighting) does 22.1 damage per round and a 2H fighter (using Great Weapon Master and Great Weapon Fighting Style) deals 26.8 damage per round. The dual wielder gets +1 AC but the 2H'er retains their bonus action. The 2H'er is expected to deal 20% more damage.
However, this is all assuming non-magic weapons. See more math
A level 5 fighter with a +1 greatsword and GWM and GWF deals 17.9 damage.
A level 11 fighter with a +2 greatsword and GWM and GWF deals 29.2 damage
A level 11 fighter with a +3 greatsword and GWM and GWF 30.4 damage
A level 5 fighter with two +1 shortswords and TWF deals 16.6 damage
A level 11 fighter with two +2 shortswords and TWF deals 27.3 damage.
A level 11 fighter with two +3 shortswords and TWF deals 29.9 damage
In summary, assuming +2 weapons at level 11, a dual-wielding fighter does 27.3 damage per round and a 2H fighter deals 29.9 damage per round. The 2H'er is only 9% more effective. This difference reduces to 1.6% if both have +3 weapons.
Finally, we get to @Maximuuus's points, where you can coat your weapons with fire or poison. While a dual-wielder has twice the weapons to coat, they sacrifice a bonus action attack in order to coat them (immediately lose out on 1d6+4 x 0.65 ~ 4.8 damage, but then deal 1d4 x 0.65 = 1.6 more damage each turn thereafter). So it's only worth it if the coating lasts >3 turns or you coat your weapons before the fight starts.
p.s. if you have >65% chance to hit, then 2H'ing is even better (40.2 damage at level 11 versus DW's 34.9). So yes, in this Larian-BG3-always-advantage world, 2H'ing is superior
Edit: @Nouri, you have to take into account chances to hit. Especially the reduced chance to hit while using GWM. It's also important to use average damages, not max damage. Using max damage artificially exacerbates any differences
Edit2: I didn't account for GWM's bonus action attack when you reduce an enemy to 0 HP. So 2H'ing definitely wins out if you're fighting a lot of small enemies
