6 video source limit?

Before I use three stereo cameras and one video game footage capture to make a complete scene. To make the three stereo captures I had to take each of the three perspectives and put two cameras in front of it one for the left eye one for the right eye.

Apparently if I use six cameras natively with one game footage source, one of the cameras just eventually stops working. I read somewhere there was a six video source limit that can be simultaneously used, but there are creative ways to get around that. How does one do that?
 
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