If I remember correctly, Johnny U and Raymond Berry practiced one route with one or both closing their eyes.
When Berry coached, his receivers were having trouble with a route. He decided to show them how to run it. He noticed the hash marks were in the wrong place. Attention to detail.
I recall Raymond Berry talking about Weeb Eubank encouraged Unitas and Berry run their routines after regular team practice...establishing that synchroncity.
I heard the story you allude to, maybe Art Donovan talked about it. He was talking about how the duo in response to local reporter set up one night in the stadium, lights out, dark as could be and ran set plays. 30-40 yds, maybe more, where neither could really see the other, but they connected as though the stadium lights were on. Forget how many sets they ran, ten, a dozen? But zero misses. And Berry had those great hands....no stickum gloves back then.