The meaning of "takes two to know"

Solution 1:

Yes it is an idiomatic expression:

It takes two :

If you say it takes two (or it takes two to tango), you mean that a situation or argument involves two people and they are both therefore responsible for it.

• Divorce is never the fault of one partner; it takes two.

(Collins Dictionary)

Solution 2:

Arnold Layne is a song about a Transvestite. Collecting women's clothes from the wash line suits him fine as he tried them on, distorted view in the mirror thinking he's a girl, yet see-through baby blue is a boy. It's not the same, a boy can't be a girl: it takes two to know that there is a difference between a boy and a girl, why can't you see? (The song was banned on some English radio stations at its debut.)

Solution 3:

I've always thought that it was a bit more existential than that, i.e. that something is not real unless it's experienced by more than just you?