There is no 6pin-atx power connector that I know of, you must mean what is usually called PCIe-power. You have no PCI ports on that motherboard, those ports are called PCIe x16.
Yes you can use an Y-splitter and detach 2 pins. It would, however, be useless as the use for the connector is to give more power to the motherboard. If you use an Y-cable it will be the same thing as only connecting the power to the PCIe-card directly. I am talking about 8-pin PCIe-power not EPS.
It is possible to do what you want to though.