To me, it's no harder than "snapping" to the central image when wearing VR glasses like the Vitures. What works for me is telling myself that the central image is further back than the two slightly unfocused images either side. Just keep telling yourself "it's further away, and it's the real one. Those two are fake." As soon as it clears up just a little, I then allow my gaze to wander around the image. It's extremely important to try this on a real monitor because small movements to the phone in your hand might get in the way. Or in my case, I'm longsighted in one eye, so while my left gets a clear image my right doesn't. I have to use a large image on a real monitor at least 2 feet away.

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.

Point each roll to each image.

There you go.