I'm not really in the know with Maximizer, but here's a try with the 1680-1080 ;
Since that would mean the screen would be split and one above the other (instead of next to one another) you'd get 840*1080 for each (correct me if I'm wrong)


REM batch file to launch 2 windows split horizontally
REM Upper Screen
[path of 1st copy of wow]\maximizer.exe --margin 0,840,0,0
REM Lower Screen
[path of 2nd copy of wow]maximizer.exe --margin 840,0,0,0


Since you'd only have to account for the split in the middle you don't need to specify what the left or right side margins are (just the top of the lower one and the bottoms of the top one).

Now this doesn't accout for the gxResolution of both screens.. so you might have to wiggle that a bit .. hope that this was useful a bit.