Congress gave up its ability to appoint a special prosecutor years ago. They could reauthorize the process, but the enabling law would be a long time in the making.
What seems to be being alleged is that Congress recommending a special prosecutor be appointed is, in this particular case, the moral equivalent of a special prosecutor being appointed. Presuming that your point is correct (? — but not knowledgeable either way at this moment), two ways in which I could envision that happening is if after the DOJ receives the request, then
1. (if the DOJ ignores or declines to appoint an independent prosecutor) Trump fires Sessions for not following Congress’ request(s) to appoint a special prosecutor, and appoints a replacement AG who will follow Congress’ request(s), on the premise that Sessions is not being respectful of Congress’ requests (or whatever) in regards to appointing a special prosecutor; or
2. the DOJ (i.e. Sessions) feels pressured under the prospect of being fired by Trump to appoint an independent prosecutor.
a variant of (1) is that Trump fires Sessions and then Rosenstein in rapid if Rosenstein were also to refuse to appoint an independent prosecutor. In theory, Trump could continue to fire AG line of succession until someone agreed to appoint an independent prosecutor or Trump gets another AG approved. This would give Trump an opportunity to clean house at the DOJ. At some point, someone in the AG line of succession would probably agree to appoint an independent prosecutor.
I am just guessing here, and my understanding might be wrong.