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I didn't say anything about heavily armored knights, and neither did Toady. He said he was using adamantine blades, so it's not really possible to infer how momentum modeled at this point, given the strange properties of adamantine. If it works by just letting you choose 2 attacks at once, then going on the current system one stab might be "easier strike" while the other "difficult strike." This might mean that one blow sticks the enemy but they get a counterattack on the other and stick you. Still unrealistic, but that would still be a drawback to making two attacks at once.
But you can't assume that it's not modeled in the game; Toady's post didn't indicate anything about that. He merely said he was testing simultaneous attacks, not which aspects or whether or not the dual-stabbing gave him advantage over other arena contestants.
Sometimes it's diffiult to visualize why dual-wielding is a bad idea. It can help to imagine the difference between throwing something at a target with one hand as opposed to throwing two things at a target with both hands at the same time. There'll be the arguments that it's not the same thing, but the principle is the same. Then there's the argument that it's not about accuracy or damage, but rather the extra attacks you get if you attack with one hand and then the other right after and so on. Again if you visualize throwing, you can intuitively see that doesn't make sense either.
Just to be clear, I'm not saying a double-stab is better, I'm just saying it's not abysmally horrible and nigh-impossible to pull off. First off, assume neither opponent is armored. Used effectively, a double-stab isn't like throwing, but more like thrusting with a 2-handed weapon. Imagine a shove: you put a hand on each shoulder blade, or one on the shoulders and one on the middle of the back, and push. Now imagine the same motion, but instead of pressing with the palm of your hands you press with the tips of 2 knives. No trying to do different actions with each hand just doing the same motion with both, spaced slightly apart. The closer the points, the more similar to using 2 hands to thrust the same knife.
More than likely, you just have the offhand shank in case your other hand gets grabbed or otherwise incapacitated, but should you find yourself in position to stab with both, you could make two nasty punctures. It's not an insane or irrational way to fight, it's just that the opportunity to double-stab doesn't come up as often.