If I'm not mistaken, you must connect all risers with same PSU that you connected to mobo. Second PSU to power some GPUs only.
It could be a serious mistake. Just let's imagine that Secondary PSU (GPU additional power) shuts down, but that GPU continues get the power through PCIe slot....
So the GPU can be burned out.
All power connectors (i.e. riser and additional power) of single GPU should be connected to the same PSU