You do not actually do more damage unless you take the Dual Welding perks! Take the first two ranks, equip 2 weapons and JUST swing the right hand. Notice how fast it swings. Then equip a shield and swing the right hand. You will notice a big difference in swing speed. I don't think you actually have to swing both weapons - you will get a speed increase with those perks.
Next, get the perk Dual Savagery. It gives +50% dmg to a power attack as long as you have two weapons equipped. So just power attack with a single weapon + 50% damage. That single perk makes the a Dual Weld player hit just as hard as a two hand warrior. For 50% less stamina. Now power attack with BOTH weapons at the same time. You will will hit for double damage on each weapon + 50%. So two Daedric Swords with max skill hit for 72 pts of dmg each. A 2 hand hits for 106. After dual savagery they hit for 72 * 2 (two weapons) * 2 (power attack) * 1.5( Dual Savage) = 432. No fancy bull crap enchants either. A two hand warrior will hit for 106 * 2 (Power Attack) = 212. A single Sword & Board will hit for 72 * 2 = 144. A dual weld character that only attacks with one hand will do 72 * 2 (Power Attack) * 1.5( Dual Savagery) = 216.
Now the formula to calculate the stamina you burn up is (40 + 2 pts per unit of weapon weight) * .75 (If you have the perk that drops stamina cost - which you had better get if you are playing a melee warrior!) Since most 2 Hand weapons weight double the single hand weapon - Dual Power Attack uses the same amount of stamina as a 2hnd power attack - but at a whole lot more damage!
The only advantage of 2 Hand is Sweep and tha ability to block/parry.
I'd say overall DPS for dual wielding compared to two-handed when all perks considered is probably in favor of dual wielding by close to 25% more in damage output per second could be a little less.
However this is somewhat mitigated in that two-handed can block and hence reduced damage taken.
Also two-handed can make use of the block tree perks. Such as the ability to block 100% more effective (probably mitigating almost all damage) and also do "power-bash" that can stagger and do 5x damage (whatever default bash is), not only that but a chance of totally disarming his opponent as well.
I forgot you can also slow down time during the blocking, which can set up ability to count-attack ( that one might only work for shields, not sure, the counter attack that is, the slowing of time should still work).
So in the end I think it evens out, sure you have to spend perks in block tree for those abilities with the two-handed, but it's something a dual wielder can't use, unless of course they go back to using a shield or using one sword and NOTHING in the other hand.
You might do a little more damage as a dual wielder of swords/axes etc but you can't defend yourself so every hit on YOU is a full on damage. It can be quite a significant trade-off, I mean it's not like a two-handed warrior can't dish out the pain as well, but they can defend too.
I was actually pretty much focused on going one handed and dual wield a lot, but this made me re-think this decision, I never thought of how much more defense I can get. Always just thought of it as blocking and only thought shields was needed for the whole block tree.
But that is not the case, the bashing etc and damage mitigation works for two-handed as well.
Decisions decisions, arghhh...