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Why add a second powered riser? I've been curios as to what these are used for.

I'm really curious to see what would happen in the long run.

Touch the single 12v wire going to your motherboard connector to see if it's hot,
It may be providing ~45 watt per GPU, or 30 amp on one wire. (rated for ~8amp)

(45w according to http://blog.zorinaq.com/?e=42 using 5970)

I personally use powered extender when going past 5 gpus.

I'll check it out when I get home, you've got me curious now. This board does allow me to run 2, 8 pin 12V ATX power. I don't have the second plugged in right now, never thought about it

Nice dual 8pin EPS plugs. If your powersupply has dual 8pin EPS connector for the motherboard then I would use it. More connectors = lower amperage = higher efficiency = less heat.

Mine has an extra molex connector right by the 24 pin power cable.
I think that having it by the PCIE slots would be a better solution, however i don't know how wide the 12v lines in the multi layer boards.