People here need to give the Scythe some more credit! Late game with maximum critical skill it becomes amazing. I've seen hits well into the 1200s in my current game.
I did some theory crafting with 2 identical endgame barbarians, one wielding the best 2hander and the other wielding the best two 1handers. In addition I'll also gave them three points into dodge and the Bracelet of Tirin for a total of 25% reduced cooldown. I also gave them maximum weapon and critical skills and a high strength score of 50. (20base+15lvlbonus+9headhunter+6strengthpots/gear)
So in the end we have:
Total DPS for 2-hander = 53.51
Total DPS for dual 1-handers = 69.08
Meteor Hammer:
Base DMG: 70 + 188 / 2 = 129
Crit Mod: 2(20/100) + 1 = 1.4
Cooldown: 4.5 * 0.75 = 3.375
DPS: 129 * 1.4 / 3.375 = 53.51
Scythe:
Base DMG: 45 + 113 / 2 = 79
Crit Mod: 3(25/100) + 1 = 1.75
Cooldown: 3.5 * 0.75 = 2.625
DPS: 79 * 1.75 / 2.625 = 52.66
Bone Blade:
Base DMG: 37 + 89 / 2 = 63
Crit Mod: 2(20/100) + 1 = 1.4
Cooldown: 3.4 * 0.75 = 2.55
DPS: 63 * 1.4 / 2.55 = 34.58
Cutlass:
Base DMG: 36 + 86 / 2 = 61
Crit Mod: 2(20/100) + 1 = 1.4
Cooldown: 3.3 * 0.75 = 2.475
DPS: 61 * 1.4 / 2.475 = 34.50
So, when not using any special abilities and assuming we're attacking at every chance possible, dual wielding comes out ahead in terms of sheer DPS. However this is not taking the enemies protection score into account. We don't really know the formula for protection, but it's clear that it removes a flat amount of damage, not a percentage. Meaning that heavy weapons will suffer less from it than light weapons.
But who doesn't use special abilities? Well, in this scenario of DPS'ing in a vacuum, due to the delay before charging actually begins, the charge time itself, then finally the additional cooldown, it isn't all that much of an improvement. Energy is a factor too, but we'll assume at this point you have plenty of energy potions. With the charge time reductions, being a fighter here is actually higher DPS than a barbarian. Also the Scythe wins out here, since it has no additional cooldown after using its special attack.
So our numbers with special attack spamming are:
Total DPS for 2-hander = 64.72
Total DPS for dual 1-handers = 78.45
Scythe Ability (Fighter):
Base DMG: 37 + 98 / 2 = 67.5
Crit Mod: 10(25/100) + 1 = 3.5
Cooldown: 3.5 * 0.75 + 1.1 = 3.725
DPS: 67.5 * 3.5 / 3.725 = 63.42
Cutlass Ability (Barbarian):
Base DMG: 36 + 86 / 2 = 61
Crit Mod: 2(20/100) + 1 = 1.4
Cooldown: .75(3.3 + 2.0) + 1.85 = 5.825
DPS: 3(61 * 1.4) / 5.475 = 43.98
So which is better? That depends on a number of things. How often are you moving, how much protection does your opponent have, are you fast enough to click with all your characters, are you backstabbing, are you using rage or bear form, etc ,etc. In the end it doesn't really matter which is better anyways, you should have both in your group. You've got 4 characters and there's only so much good gear to go around, it wouldn't make sense to have two characters whom need the same type of gear, one or both of them would just end up subpar.