It would be feasible to construct a ciphertext that encodes two plaintexts, depending on which document is used to decrypt it. To cover all possible plaintexts

It would be feasible to construct a ciphertext that encodes two plaintexts, depending on which document is used to decrypt it. To cover all possible plaintexts, you'd need 576 word indexes, representing each pair of letters AA, AB, ..., ZZ. It might be hard to find a number i such that the i'th word of the Declaration of Independence begins with Z and the i'th word of the document you are using as the other key begins with Q, etc. But with some license to tweak both plaintexts, this seems within the capabilities of a motivated 19th century prankster. That said, I can't think of any legitimate reason to use this encoding, so it still points to a hoax or prank.