So why would you run multiple video cards? The main reason people go for it is its price to performance ratio. It varies from card to card, but in some cases, running two mid-range cards is slightly cheaper than running one comparable high-end card. You can get the same performance for a few bucks cheaper. They're also ideal for multi-monitor or high-resolution gaming. Plus, they look sweet, and some people just want to make their rig look as baller as possible.
So that sounds awesome, right? Unfortunately, running multiple cards comes with some cons. For example:
Two video cards sitting closely together in your case will draw more power, produce more heat, and produce more noise. If you're concerned about any of those things, SLI and Crossfire may not be for you.
Not all games support SLI and Crossfire. This depends on your video driver, not the game itself. NVIDIA and AMD often update their drivers to include multi-GPU support for new games, but if one of your games isn't supported, you'll either have to deal with one GPU or tinker with your driver settings to get the game working yourself.
SLI and Crossfire can sometimes cause a phenomenon called micro stuttering that makes the video look a tad choppy. It can be particularly aggravating to some people, especially at lower framerates.
In short, using two video cards may require more attention and tweaking on your part, whether to compatibility issues, heat, or just dealing with micro stutter.