Not sure if I'd actually implement this, but it's pretty simple, and would work with Holmes Basic, the weapons speed system I detailed previously, or any combat rules that use reverse Dex order for order of attack. Suppose you want to give fast characters, or characters with shorter weapons than their opponent, two attacks in one round. How could you do it?
Your Dex score (plus opponent's weapon length, if longer than your own weapon) is the moment of your first attack. Subtract 10 from this number; if the result is more than 0, that is the moment of your second attack. Thus, average mercs will only get one attack, but a guy with a dagger fighting a guy with a great sword will probably get a second attack after the opponent's attack. A guy with Dex 18 could even possibly get two attacks before a really slow opponent can strike.
Oh, and a second attack with a polearm is at an additional -10 penalty... so only one attack per round, unless the situation is really phenomenal.
(I picked 10 because it's easy to eyeball. Dex <= 10? Probably only one attack/round. Dex > 10? Two attacks; drop the first digit to do the subtraction. Unarmed monk with Dex 18? Maybe even a third attack, if the opponent is using something longer than a shortsword.