countersign

Short definition from Wiktionary

Noun

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

Verb

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