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.