At identification level IDN3, why can two pieces of IE.b evidence replace one piece of IE.c evidence?

Because two independent documents that each carry an identifier and security features corroborate each other: forging both consistently is much harder than forging one, which gives roughly the confidence of one fully recognised and authenticity-checked document.

IE.c means a document that is recognised in its context and checked for authenticity, typically a valid passport or ID card. Not every applicant has one at hand in every process, and not every channel can verify one. IE.b evidence (a document with an identifier and security features, but not fully verified) is weaker on its own.

Combining two of them changes the picture. An impostor now has to produce two documents from different sources whose names, dates and numbers match each other and the claimed identity. Each additional independent source multiplies the effort and the chance of an inconsistency. The same idea appears in NIST SP 800-63A, where identity proofing can be satisfied with combinations such as "two pieces of STRONG evidence" or "one piece of SUPERIOR evidence".

Note the limit: at IDN4 no such substitution exists. IE.d evidence (state-recognised, with physical features of the person) is mandatory, because only that ties the document to the body of the person presenting it.

Go deeper:

• NIST SP 800-63A-4: Identity Assurance Level 3 — the evidence combinations (FAIR, STRONG, SUPERIOR) accepted at the top proofing level.