I usually only game on one screen, but have all my extras on the second. Like temp monitoring, winamp, webpages if needed.
Never tried running a game across multiple monitors, I just like being able to max a game out and have it pretty on one... Really thinking I'm gonna get a nice 1440 or 4k monitor after Christmas.
That seems like the best idea if having high detail and frame rates is the objective to gaming. Spanning across a higher resolution will definitely tax your GPU on the VRAM, but considering the 900 series is almost able to run modern games in 4k at respectable frame rates on one GPU alone, it won't be long til nothing is held back.
4k res, on multiple 4k monitors, max settings, hq textures, mods galore, all at 60+ fps, bring it on.