I donít have the setup to test this, hence the question: Letís say that a visitor has two monitors - if you try to get visitorís screen size with JS code (window.screen.width and window.screen.height) what will the result be? Is it just the size of the area of their browser, data for only one monitor, or data for both monitors combined, or..?
It will be the size of the current application's portion of the desktop. Thus, if you have two monitors with a 1600◊1200 resolution, it is not going to show 3200◊1200, but instead only the resolution for the monitor in which the app currently runs. This can, certainly, as has already been mentioned, be different between the two.
Actually ... Just did another test. If you have it set to allow windows to span the two monitors (thus maximizing over both monitors instead of the normal one) it will show the combined total, which in this case can be completely different than the two resolutions combined, as it will take into account dead space from adjusting the desktops height wise.
It also depends on the video card, whether you have only one or two ... and (if one) whether it is capable of treating both monitors as one single desktop internally.