countersign

noun

1. A second signature added to a document to affirm the validity of the signature of the first person.
2. The response to a sign or signal.

   “Only those who give the countersign may enter the Masonic Lodge.”

verb

1. (transitive)To sign on the opposite side of (a document).
2. (broadly, transitive)To add a second signature to a document, affirming the validity of the signature of another person.