I have two PSU's connected to my B250.  The first has the CPU line ran to the board. It also has a molex running to the first set of PCIe slots and if I remember correctly, you are supposed to use the same PSU for all three of the molex slots by the PCIe slots - I'm using the same PSU 1 for that.   Then I have a second PSU that is plugged into the #2 PSU slot on the board and runs to gpu's not connected to the first psu.