if you are talking about the arena, or against monsters, i'm pretty sure you need three times their quickness to strike twice per round against them. of course, we don't really know the quickness of monsters, so this arguement is only really valid for the arena.
however, i can say that, on the level you first see a monster, almost every monster will have a quickness somewhere between the leve you're on and twice that (i can say this because i've played char's with quick=lvl, and never hit first against monsters, and i've played a char with quick=2*lvl, and hit first against almost every monster i see when i'm calamaty-free). so, to have three times their quickness, you need between lvl*3 and lvl*6... very impractical beyond about level 5!
as for pvp's, if that's what you were referring to... i haven't looked in depth at the time needed for each player. some people have... i believe thomas and/or tet, or possibly some others, i don't really remember. but depending on the speed difference between char's, and the number of hits that will happen before someone dies, it's quite possible to double-hit them in one round... if you've got above-average quickness.
--jason
