Not seeing eye to eye, means to disagree on something, to not have the same opinion, and for it to be a problem.
She doesn't speak to her uncle any more, they haven't seen eye to eye since he slapped her.
In fact, they don't see eye to eye on anything.
It's a social behavioural thing, when we're sincere, we look in people's eyes, when we're angry/hurt/offended, it could make us look away.
"Eye contact" means trustworthiness, sincerity, gentle assurance.
We never used to get on, but after we had a long discussion, we now see eye to eye on more things.
It means, "we share an understanding" and it's a literary description of social behaviour.
Much 'in the same vein' as, "he turned his back on me", which means he became disloyal.
I'm not sure if it's idiomatic or not...... It might be... Jura? You're the expert.