In a 2 Power Supply Setup I have all my risers connected to one PSU.
The GPU's can be spread over 2 or more PSU's but you should avoid connecting a single GPU onto multiple PSU's (6/8 Pin connector)


It is all DC Power so as long as you connect to the Same Ground you will have no problems, make sure you have proper grounding and use cables that are 3 prongs (Wall Grounding, Extension Cords, Power Strips and Power Cables)