This may seem elementary to some folks but I'm just concerned that 2 cables would provide too much power and ruin the card if in fact I'm only supposed to run 1 cable with some sort of splitter. Documentation seems more than limited regarding this specific question so any direction or help would be much appreciated, thanks!
Because the potential power draw of the GeForce GTX 580 is high both a 75 Watt 6-pin and a 150 Watt 8-pin PCI Express Supplementary power connectors are required.
I don't see how the graphics card can receive too much power. The graphics card will only draw as much power as it needs. 33 Watts when idle and up to 226 Watts when gaming.
Both cables are mandatory otherwise the card won't function properly, if at all. This is needed to distribute the electrical current over more wires/conductors to prevent the wires themselves from overheating and melting.
So you should be using two modular PCI-E Cables (6+2 pin). On one cable you will connect only the 6-pin portion to the 6-pin PCI-E connector on the graphics card and leave the 2-pin portion dangling/unconnected. On the other cable you will connect all 6+2 pins to the 8-pin PCI-E connector on the graphics card. This ensures that the graphics card will be able to receive sufficient power.