Is it correct to say "You which know the secret of my heart"? If yes, what is the difference from "You who know .... " ?
Adding which makes "You which know the secret of my heart" a sentence fragment.
This would be the proper way of writing it:
You, which know the secret of my heart, verb blah blah.
Note the added commas. Also, using "which" instead of "who" is archaic. That's probably a quote from the King James Bible.