How to calculate binocular overlap
I'd recommend looking at image stitching algorithms. Even though you don't want to actually combine the images into one, that's still essentially what you're doing.

You might even be able to use an off-the-shelf stitching algorithm to get a rough measure. e.g. If the stitched image is 100 pixels wide, and your two source images are 80 pixels wide, then horizontally you have a 60 pixel overlap.