Wouldn't it take much more power? And how would that work, if I only had 1 screen, how could 2 work? I have 2 if you can count integrated as "one" and it won't work with a physical GPU. So is 2 graphics cards or more for multi monitors?
If you get 2 identical (manufacturer doesn't matter) Nvidia cards (SLI), you can connect them with an SLI bridge and if you get two AMD cards (these don't have to be completely identical), you can connect with a CrossfFire bridge. You also need a CF/SLI compatible motherboard for this.
This allows the GPUs to work together, giving your PC more graphics performance. This would also require a lot more power from the PSU. Both GPUs can also be utilized with a single screen.
The problem with this is that 2 GPUs doesn't mean 2 times the performance. How much the 2nd GPU actually helps depends on the optimization of each particular game; Some games utilize this well, other don't do it at all.
Multi-GPU setups are for situations where a single fast GPU still is not fast enough. There are four typical cases for that:
1- gaming at 4k resolutions
2- gaming at multi-display resolutions (ex.: 5760x1080 or 3240x1920)
3- gaming at beyond-60fps for people with 100+Hz screens
4- gaming in stereo-3D
In most of those cases, running everything at Ultra details.
People with multi-GPU setups rarely care about power-efficiency... or even cost-efficiency. They want more performance than what a single GPU can provide and going multi-GPU is the only way they are going to get it.
Setting up a Crossfire/SLI configuration while it does not double the performance it will increase the performance in most game's. With some games it will let you get a 60+ frame rate while with others it will let you set better detail.
For me it lets me run a Surround setup on my three Asus 27" monitors and with most games I can run them at max/ultra settings.