Kenjib, I was all prepared to disagree with you, but then something struck me. With two weapons, I often feel as though one is the set-up move and one is the strike -- with two sticks, I'll get into a technique that goes check-STRIKE-check-STRIKE, where the check is still gonna hurt, but mainly it's there to keep the guy's arm down while I whack him in the head. With the double spear, what I really benefit from is not having to reset my weapon as much. Instead of stabbing, pulling back, and stabbing again, I can stab (a glancing strike across the side of the throat) and then immediately move into a side-swing, without having to reset the weapon. The end result is a shorter time period between attacks -- which in D&D, translates to an extra attack.
So I think it's 50/50. 50% set-up, checking, or defense bonus from the other weapon, and 50% additional attack potential.
Potential house rule which would definitely overpower two-weapon style:
Each round you use two weapons, choose what to do with your off-hand weapon:
1) Gain an AC bonus from parrying -- +1 for Tiny, +2 for Small, +3 for Medium. Take off-hand fighting penalties with your attacks.
2) Set up an attack -- you gain a +2 bonus to one attack that round (your first melee attack), although you can do nothing else with the off-hand weapon. Off-hand weapon-use penalties do not apply.
3) Two-weapon as normal.
4) Do nothing.
In order to not totally overpower two-weapon fighting that way, I'd allow all of that with a shield as well.
Probably too much. But that's how I use double weapons or paired weapons in real life...
-Tacky