How do I use a quote that contains two authors words at once (MLA citation)?

The simplest way to do this is to quote the page as you have it, pointing out the as Silver says part and for your in text citation to just cite the McKibben book. Anyone who checks your references (uming they're done properly) will be able to tell that this is you quoting McKibben quoting Silver. It might also be less confusing to just paraphrase or to say something like "as Silver is quoted in Name of Book...." Be sure that whichever way you do it you use McKibben as your citation author since he's the source.