For the sake of clarity, let's see what exactly the energy costs are at this moment.
Let's assume a Two-weapon combat skill of 100. A reasonable assumption, because the skill is easy to raise through training. Let's also assume that the lowest weapon skill the player has in the two weapons he is wielding is moderately high, say 8 to 9. This means he can attack with one weapon for some 800 energy points (and attack twice for 1600 EP). When dual wielding, a double attack will cost him 1200 energy points.
Now let's also assume that the dual weapons are both equally good, which is reasonably possible as there exist good candidates like bloody daggers and phase daggers. So his damage per second rating is then increased by a factor of 4/3, that is, +33% if he chooses to dual wield. The higher your weapon skill, the worse dual wielding becomes compared to fighting one handed (because the off-hand attack always costs a fixed 400 EP). At zero skill dual wielding increases your d/s by 43% and at grand mastery it increases your d/s by only 20%.
Actually, now that I think about it, letting the energy cost of your off-hand attack depend on weapon skill instead of a fixed 400 EP seems best to me.