If you want to run surround with an extended desktop or "accessory display", then you need to get an additional video card. It doesn't have to be the same as your current video cards, any card capable of acting as a PhysX card will work, I believe. If you don't have an open PCI-E 16x slot however, I believe you are out of luck.
Visual aid from NVIDIA's site:
Your 580s -->
Any Other Card Card -->
(from : http://www.nvidia.com/object/3dv-system-requirements-surround-technology.html)
As far as any unconventional methods such as a switcher, I think it would be easier to just disable surround and enable single-screen SLI, but thats just me.