Try this (This is using values that I have working for me):

First, back up your existing xorg.conf to something like xorg.conf.BAK

Go to the Monitor section for the 1440p panel (Monitor1) and either delete or comment it, try pasting this in it's place:

##########################################################
Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "DFP-3"
    DisplaySize     597 336 #NEW
    HorizSync       88.8 - 88.8     #MODIFIED
    VertRefresh     59.5        #MODIFIED
    ModeLine       "2560x1440" 241.500 2560 2608 2640 2720 1440 1443 1448 1481 +hsync -vsync        #MODIFIED
    Option         "DPMS"
EndSection
############################################################


The next thing for you to look at is the two Screen sections at the bottom, specifically the first one (for Monitor1):


############################################################
Section "Screen"
    Option         "Stereo" "0"
    Option        "NoLogo" "True"  #Uncommenting all these, so it uses the custom settings
    Option        "UseEDID" "False"
    Option        "UseEDIDDPI" "False"
    Option        "UseEDIDFreqs" "False"
    Option         "metamodes"      "2560x1440"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "ExactModeTimingsDVI" "True"
    #Option         "metamodes" "DFP-3: 2560x1440 +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection