I have a similar configuration, dual screen but the main screen is on the right and mIRC is ran maximized on the second screen (left). It starts correctly maximized on that screen.
I think only the 'main' item from the windows section is relevant here, I have main=-983,1440,158,900,2,1,0 if that helps.

#mircscripting @ irc.swiftirc.net == the best mIRC help channel