I'm skeptical that this would come up.
While an NDA would often refer to a confidential document that can't be disclosed, normally, the obligation of the party to the NDA would not itself be kept secret because this would impair, as a practical matter, the capacity of the party to the agreement to perform the agreement.
I suppose the NDA might specify the documents not to be disclosed, in general, while the confidential schedule to the NDA might contain a list of all of the specific secret documents that are out there by date and title. But even then, preventing the person charged with performing the contract from knowing the scope of their obligation is at a minimum bad practice, even if it is enforceable.
It could depend also on the complexity of the confidential document.
But again, I've never seen an NDA that denies to a party to the NDA access to the scope of his or her obligation.
Then again, it is routinely the case, that obligations are set forth on a single piece of paper (or a single electronic file) that is only maintained by one party. Usually the party in possession would have a duty to make the document in their possession available upon request (or at least to clarify what it says upon request), but the other party might not be the custodian of the document (e.g. an original promissory note).