I never tried it as I always use one big good quality PSU per rig but as far as I remember reading about it long ago you have to feed each card from the same PSU on both sides.
This is because the molex going into the riser and the 6/8-pins on the GPU are not connected to the motherboard (in terms of 5V and 12V) so you're not connecting multiple PSUs to each other which would be a problem.

But wait for others who have first hand experience.