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
Based on the text in the playtest material: "All the damage of each of these attacks is halved"
The first word is "all", and it's "of these attacks", so you'd have to mentally allocate the DS and SA damage to
not be part of the attack to arrive at adding them on without halving them. Which seems a bit willful and convoluted . . .
However, I expect the wording and final version of dual wielding could well change.
I'd much prefer to avoid 3.5 (and especially 4E's) assumption that we all want to be lawyers and argue endlessly about what an "attack" really is in canonical form within the game. Please WotC, in the more refined versions of the text, clarify statements like "all damage is halved" with some examples including combinations with other abilities. I want to play D&D, and I want the designers to spend a little more time spelling out what they meant so I don't need to.
Edit: DPR doesn't capture important considerations such as the spread of likely damage. The two-weapon attacker has a wider spread of outcomes where they make at least one hit, doing a little damage a lot more often than the two-handed user. This makes no difference when fighting creatures with large amounts of hit points. In the playtest however, a lot of creatures will end up on the edge of being taken down within 1 or 2 hits. That makes amount of damage done, especially at low levels, less of a concern than simply hitting. However, two weapon fighting, as written, will scale to high levels badly unless it is given further boosts to offset a lower DPR.