I have a 750W Corsair and a 1000W EVGA in one rig, combined with this http://www.add2psu.com/

Haven't had any problems yet.

I thought the same about the riser and GPU but what ended up working for me is:

All risers, cpu, mobo, sata on one PSU and all the GPUs on the second PSU.  If I didn't do it this way the cards in the risers connected to the second PSU would not be recognized.

-phil