I think you'd need to edit the .ini file for Civ V to run it in windowed mode and use a custom resolution. Found this topic that should hopefully help: http://forums.steampowered.com/forums/showthread.php?t=2874827

If you have two 1920x1080 monitors, you'd set the X value to 3840, and the Y value to 1080. You might also make them slightly less than that, to account for the window border. Maybe X=3820 and Y=1040

Hope that helps!