I can think of one way. Using a laptop, input both streams into Bino (possibly, I think the streams can be mapped in from Bino Ream Me: To play live video from a webcam or TV card, you can set up a streaming server using ffserver (part of FFmpeg) or vlc, and then give the appropriate URL to Bino. You can use multicast to stream the video to multiple systems efficiently. ).
Then output both streams via HDMI frame packing or other method. However, note that the cameras would not be in sync unless you were using a 3D camera or two genlocked cameras.
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