One issue you are likely having with your 5870 is your frame buffer. Assuming you have a 1GB card, at that resolution, 1GB cards sometimes run out of video ram when turning up the settings. Adding a 2nd 5870 will not solve this issue.
The end result is you will end up with higher average FPS with a 2nd 5870, but you'll sometimes have a higher minimum FPS with a single 580, or even a 480.
Often times it's the minimum FPS that make a game feel more playable, rather than the max. It's a tough call. If you are willing to turn down a few of the visuals when you play a game that runs into a buffer issue with the 1GB 5870's, you'll end up saving money and have higher performance most the time.