I definately prefer two (on windows/linux at least) as without special software, you just end up using the entire 30" (maximize button and all) and then it just gets too inefficient with respect to mouse movements. With two monitors, I can maximize a document in one screen (and the window will behave like a maximized window...no accidental dragging or resizing) and do what I want on the other screen. A new feature I have actually been enjoying is making something full screen on one screen (killing any toolbars/menus/etc) and still having the taskbar and non-full screen apps open on the main screen.
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Can you actually look at two monitors at the same time? I assume the answer is no, so what can two do that one can't?
You don't look at both...but you rarely look at the entire single monitor either. It is perfect for having a document you are working on and a source side by side...or a code editor/excel and a reference page. At work I use a monitor with my laptop next to it...email is always open on the laptop screen and primary work on the main screen. If I start doing a lot of back and forth between two windows...just move one to the laptop screen.
Nifty bonus trick: Set the right screen to be slightly "lower" than the left screen in the display settings. I do this because my laptop is actually a few inches lower and it makes the mouse jump less when you cross. Here is the real bonus though (and why I still do it slightly with level displays): your mouse will still "catch" in the top right corner of your left screen...allowing you to twitch to the corner and click the x on a window without ending up on the wrong screen.
















