Dual monitor setup failed

I am running MTL with my primary display hooked up to the DVI port of my ATI 5870. My second monitor is hooked up to one of the mini-display ports (mini display port to display port cable) and gets no signal. Tried forcing monitor detection from the Displays control applet but nothing happened, turned off secondary monitor, unplugged cable etc nothing.

I know the cable works because I'm currently using it to output video from my Lenovo T430 laptop.

Both monitors are high end HP ZR2440w.

Has anyone had the same problems when setting up multiple displays on a Mac?

Only the monitor connected to the DVI output shows up in the System Report but that makes sense since monitor detection didn't work. Are you implying that both mini display ports on my graphic card are dead?

Problem fixed. So it turned out detection didn't work until disconnecting the primary monitor connected to the DVI port. Now it works fine.