Another method is parallel eye though it's bit more complicated than the finger.
Zoom the images in front of your eyes so that they are little less than same width as your eyes.
Try to look behind/beyond the image, as in let your eyes loose/relax. Don't stare at the images instead see in that direction as if you are day dreaming.
Images will overlap at one point. If they don't completely perfectly overlap, reduce the zoom.
Once they overlap, focus the overlapped image.
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Method 2: Use kitchen towel rolls
After putting those images in front of you, make a binocular out of used kitchen rolls.
Zoom the images in front of your eyes so that they are little less than same width as your eyes.
Try to look behind/beyond the image, as in let your eyes loose/relax. Don't stare at the images instead see in that direction as if you are day dreaming.
Images will overlap at one point. If they don't completely perfectly overlap, reduce the zoom.
Once they overlap, focus the overlapped image.
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Method 2: Use kitchen towel rolls
After putting those images in front of you, make a binocular out of used kitchen rolls.
Point each roll to each image.
There you go.