You are combining two sentences, but the subject is still singular so the verb should be 'has'. If you said "Both" then you would have a plural subject and the verb would be 'have'.
It is only 1 person, the witness OR the policeman, so HAS. If it was the witness AND the policeman, it would be multiple people, so it would be HAVE. (Then the sentence also has to be changed to make it correct)