primary screen is flickering

Where is the display output coming from?

The full spec of your system would be a good starting point. We can't see what you've got sitting in front of you, so you'll have to give us the details. ;)

One of the things I've run across in multiple video card systems with multiple monitors is the interaction between the cards themselves and the types of interference that happens with common power connections. One situation was from weak capacitors on one card not filtering the power supply voltages that were being affected by the other card. Another case was when the video cards were mounted right alongside each other and were interacting and causing display flickers and interlace blanking. Things are much better with LCD displays than they were with the old CRT types that had huge magnetic fields that really created a lot of hum bars and tearout patterns that made viewing very difficult and led to huge problems with eye strain. If possible, locate the video cards as far away from each other on the system board and even a reorientation of the monitors themselves can reduce some effects. Power supply problems are tough to sort out and when the video cards themselves are at fault, having a sharp tech add some extra filtering will go a long way in preventing power source interactions.