reflection and compositing trouble

Ok i got to mirror one behind the other . each one on different layer.
I set up render pass so as to get the first mirror rendererd first then the second rendered on the second layer. unfortunately when i render, the first render fine, but the second render with a black square that is the first mirror back side. but it should not be there because it s not on the renderred layer.
don t know if i m clear…:spin:
any help so as to get ride of the back of the mirror ?