RE: DELL NVidia Quadro K600 with dual monitor, how to set primary to DP?
You should disconnect the monitor from the DVI. Restart with only one monitor connected to the DP. The operating system should then make DP primary. Then, attach the second monitor to DVI. Use the operating system to make the DVI secondary.
To clarify, the operating system is on and running when you attach the second monitor to DVI and then configures it to be secondary.