Simultaneous delivery of spirit messages through different mediums with a request to forward them to the right person. The idea, originated by the communicators themselves, was to disprove the suggestion that messages were merely the working of the medium's subconscious mind.
The earliest instance of cross-reference is registered in E. W. Capron's Modern Spiritualism (1885) from February 12, 1850. The medium was a Mrs. Draper. A large company was divided into two groups and sent to different rooms. The spirit of Benjamin Franklin purported to be present and spelled out a message telling the company not to move. The same message was then spelled out in the other room with instructions to go and compare. This method of communication was called "spiritual telegraphy" and was soon practiced between New York and Philadelphia or Washington, D.C., and between Baltimore and Pittsburgh.
A deceased sister announced herself to Robert Hare at Cape May, nearly a hundred miles from Philadelphia. Hare asked the spirit to go to Philadelphia and ask Mrs. B. Gourlay, a medium, to get her husband to go to a certain bank and inquire about a certain bill. On his return Hare found out that Dr. Gourlay had received the message and the bank testified that he came to inquire about the the bill.
Bradley, H. Dennis. The Wisdom of the Gods. London: T. Werner Larvie Ltd., 1925.
Hare, Robert. Experimental Investigations of the Spirit Manifestations. New York: Partridge & Brittan, 1855.