You should have only one Screen section and one Device section, but keep the two Monitor sections. Change their Identifiers to whatever 'xrandr' reports for the monitor names (ie. LVDS1, VGA1).
Then, in those Monitor sections, put in the PreferredMode for each monitor. Something along the lines of this:
Option "PreferredMode" "1366x768"
Option "PreferredMode" "1680x1050"
You should get rid of the Modes line in the Display SubSection, too.