Hardware question: 2x gfx cards in one machine

I have a Dell workstation with a Quadro 4000 card in it. This has been good for CAD work, but we are moving into VR with the Vive and Rift. I have therefore added another graphics card, which is a GTX980Ti.

I only have the one monitor, and I’d like to keep it that way if possible (my desk is only so big!). The monitor has several inputs. I have my Quadro card connected via DVI and the GTX card connected via Displayport. I can now switch between the displays using the monitor’s own controls.

When I launch Unreal Engine, how do I ensure it only uses the GFX card (connected via displayport)? At the moment, it is using my Quadro card which I can see by using an Nvidia monitoring tool. The GFX card just sits there idle while the Quadro card is screaming away trying to display even basic things.

I assumed that dragging the Unreal Engine window to the displayport screen would ensure it only uses the GFX card, but it still uses only the Quadro.