This is what I could find. Was curious the other day and was looking into it. Not sure if 100% correct, but seems close enough to work with.
Dual Wield % for Monks: (LVL + DW Skill) / 400 :: [78% at Lv60]
Double attack: DA skill / 400 :: [62.5% at Lv60]
Double attack affects offhand at 150 skill (ie. Lv29+)
Sources:
http://wiki.project1999.org/index.php/Game_Mechanics
http://www.project1999.org/forums/sh...ad.php?t=37145
http://www.thesteelwarrior.org/forum...hp/t-2355.html