From my experience most right handed people prefer the main monitor in front of them (mostly slightly to the left) with the other monitor the same size to the right. Not enough left handed people around to figure out a trend there. One monitor above the other isn't appreciated at all (requires too much head movement with the neck). At lot of people are using their secondary monitor also for watching streamed video (so it should have that capability).
When using Poser the posing window is generally on the main screen with the library on the secondary screen. It's often too much work to move the posing window over to the other screen so when rendering (and not using all available threads) I tend to use the secondary screen for some text editing (story board writing).
In my work I often do the work on the main screen and use the secondary screen for whatever reference material is required (quite often multiple overlapping windows).
After working with different monitor setups and sizes for a long time I prefer two equally sized monitors side by side of 22" or 24" (wide screen). Multiple bigger screens mean too much movement required to actuall see what's on them. One big screen just doesn't work for me (you still tend to maximize the window defeating the purpose of the big screen). More than two screens might be great for games but it doesn't really add anything for anything else (I tend to lose things on the screens and have to start searching, so I didn't use the third screen when I had it available).

So in short a second screen is always an advantage when it can be placed next to the existing one and try to stick to the same size (no need to use the model of screen).