In order to make OpenGL create windows that are actually the indented size and not the (very) wide width of the dual monitor screen, add this line to your Screen section in xorg.conf:

Option "Metamodes" "DFP-0: 1920x1080 +1920+0, DFP-2: 1920x1080 +0+0; 1280x1024,1280x1024; 1024x768,1024x768; 800x600,800x600; 640x480,640x480; 1920x1080,NULL;  1280x1024,NULL; 1024x768,NULL; 800x600,NULL; 640x480,NULL;"

Edit it accordingly to include the modes you want. Note that it is the WIDTHxHEIGHT,NULL modes that enables OpenGL to create the windows with the correct desired size.