Depending on how your DM interprets the two-weapon fighting rules effects on Deadly Strike and Sneak Attack two-weapon fighting can deal more or less average damage per round than using a two-handed weapon.
The following formula use a 5th level Fighter with 17 Str(+3) attacking a target with an AC of 17 using Deadly Strike for full damage.
Two-handed weapon(Greatsword)
.5(6.5+3+9)=9.25
Two Weapon Fighting(2xRapier/Scimitar/Shortsword and DS and SA are halved)
0.5((3.5+3+9)/2)+.25((7+6+9)/2)=6.625
Two Weapon Fighting(2xRapier/Scimitar/Shortsword and DS and SA are not halved)
0.5*((3.5+3)/2+9)+.25*((7+6)/2+9)=10
At this point there seem to be only two major benefits of two-weapon fighting:
1) if you're fighting lots of very low hp monsters
2) if you have non-damaging on-hit effects.
There aren't a lot of examples of (2) right now, the major one is that a fighter with two CS dice could knock prone or knock back two different monsters in one round with 2WF. If more status effects are introduced 2WF will start to look better.
At this point there seem to be only two major benefits of two-weapon fighting:
1) if you're fighting lots of very low hp monsters
2) if you have non-damaging on-hit effects.
There aren't a lot of examples of (2) right now, the major one is that a fighter with two CS dice could knock prone or knock back two different monsters in one round with 2WF.
Imo the fighting styles should not be balanced around doing equal damage but around the better fighting style requiring more feats to become effective.
No investment -> sword and shield is best
slight investment -> Two handed weapons do more damage with a little loss of AC compared to shield.
big investment -> two weapons should do more damage than two handed weapons without additional penalties.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.