The subpoena to Sessions is to produce documents, not himself. Subpoena gives him a reason to obtain documents responsive, or in the alternative, tell Congress he has none (recused), and transfer responsibility to RR for production.
At some point, the Gordian knot used by RR (call something "sources and methods" and refuse to produce in order to protect his own disqualification and involvement) can be cut by Sessions notice of the unethical conduct, and removing RR from the chain of command in that case.