[Solusek]

Well, maybe 0 speed factor weapons only add an extra attack when they're actually wielded in the off-hand? I really don't know, but as I said, I tried it with various 0 speed factor weapons, and they don't increase my attacks per round, like Scarlet Ninja-To or Kundane do for example.

Edit: I did some excessive testing, and I'm absolutely sure that speed factor 0 doesn't necessarily mean +1 attack per round.

Examples:
Wielding Belm or Scarlet Ninja-To ONLY => 7/2 attacks per round
Wielding Kachiko's Wakizashi or Usuno's Blade ONLY => 5/2 attacks per round
Dual-Wielding Belm AND Scarlet Ninja-To => 5 attacks per round(normally 11/2, but
unbuffed attacks per round are capped at 5)
Wielding Belm + Kachiko's => 9/2 attacks per round
Wielding Belm + Usuno's => 9/2 attacks per round
Wielding Usuno's + Kachiko's => 7/2 attacks per round.
Wielding Usuno's + Spectral Brand => 7/2 attacks per round
Wielding Spectral Brand + Kachiko's => 7/2 attacks per round
Wielding Spectral Brand + Scarlet Ninja-To => 9/2 attacks per round


Testing was done on a multi-class Fighter(18)/Thief(21). I'm pretty sure that I will gain another 1/2 attack per round later, so just add that to the results if you want to. Another interesting thing you'll notice at this point is that you can reach the unbuffed attacks per round cap with just one weapon that grants an additional attack per round. There is no benefit from dual-wielding two weapons that each give +1 attack per round at max. warrior level. A Blade might still benefit from it though, since his base attacks per round are lower.


So long,
Sol