Getting best performance from two graphics cards

I have seen comments from people using two graphics cards, such as “I use one for OpenGL, and the other one for Cycles rendering”. I have two graphics cards supplied in an HP Z210 workstation; a Quadro 400 and a Quadro 4000. The 400 can’t do Cycles (compute capability 1.2 only, needs 1.3+), so I’d like to use the 400 for driving my two monitors, and use the 4000 only as a Cycles ‘engine’. However, as soon as I start a GPU render (with just the 4000 selected in Blender System Preferences) my monitors’ response slows down, in any application. After a minute or two Windows asks me if I want to turn off some interface features!

So is this normal? In Blender, how can I actually setup the 4000 only for GPU rendering (not other duties), so it doesn’t interfere with the rest of my system while its rendering?

You also have to connect your screen to the Q400 if this isn’t obvious, else you can tell Blender 1000 times to use the Q4000 for rendering the display will still be rendered on the Q4000.