I’m using a Blackmagic Intensity Pro (is that what you mean?)…
No problems using Nuendo 6 or 5 to drive my big monitor through HDMI, and simultaneously the talent monitor through composite. Both versions of Nuendo also recognize the drivers, so I can choose different sizes / video frame rates within N’s Device Setup. Makes it handy when I’m working on multiple projects with different original video sizes.
If you’re connecting via component, check all three connections. Otherwise you might want to suspect the drivers or card.