Eyefinity works because I just built a rig with Two Radeon HD 7870's with a resolution of 3240x1920 across 3 monitors.
I'm not a Radeon guy
and these are the first Radeons I've used since the HD 2800's which I used a long time ago. (mainly because I like Nvidia cards better, and I could code/compile my own drivers for Nvidia cards
It could be a video driver issue, or a setting in the driver, or even a conflict with another driver.
I didn't use the newest driver, and I downloaded a couple and tried them all to see which one worked and which one had problems.
Unfortunately, I pulled the two Radeon HD 7870's out and replaced them with Two Nvidia Geforce GTX 660Ti's, so I don't know which version worked
. Just have to try a couple I guess.