Q: adding a second monitor to macpro 2012

Does your GPU have more than one monitor port? If so you can try one of those instead. HDMI and VGA should work. Or get a new GPU with more than one dual-link port.

You Mac Pro can handle three displays.  It has one DVI port and two Mini DisplayPorts.

Connect the first to the computer's DVI port.  Connect the second to one of the Mini DisplayPort with a Mini DisplayPort to DVI adapter.

If the display's resolution is greater than 1920 x 1200, use a dual-link adapter.

<http://store.apple.com/us/product/MB571Z/A/mini-displayport-to-dual-link-dvi-ada pter>

Otherwise, you can use a single-link adapter.  If you will never use more than two displays, you can use the Apple passive adapter.

<http://store.apple.com/us/product/MB570Z/B/mini-displayport-to-dvi-adapter>

For three displays, all adapters must be active, such as:

<http://www.accellcables.com/collections/adapters/products/ultraav-mini-displaypo rt-to-dvi-d-active-single-link-adapter>