Literally, in the law, that which is yours. Yours has an emphatic force: of which you claim a monopoly. See Joh 7:49.
It is written (γέγραπται)
The perfect tense: it has been written, and stands written. The common form of citation elsewhere, but used by John of the Old Testament scriptures only here. His usual form is γεγραμμένον ἐστίν, the participle with the finite verb, literally, it is having been written.
The witness of two men
See Deu 19:15.
The Father - beareth witness of me
Thus there are two witnesses, and the letter of the law is fulfilled.