countersigning

/K AW1 N ER0 S AY1 N IH0 NG/
verb
  1. 1

    To sign on the opposite side of (a document).

  2. 2

    (by extension) To add a second signature to a document, affirming the validity of the signature of another person.

Source: https://en.wiktionary.org/wiki/countersign, https://en.wiktionary.org/wiki/countersigning