From Longman Dictionary of Contemporary Englishcosignatoryco‧sig‧na‧to‧ry /ˌkəʊˈsɪɡnətəri $ ˌkoʊˈsɪɡnətɔːri/ noun (plural cosignatories) [countable] SCLone of a group of people who sign a legal document for their department, organization, country etc Both cosignatories must sign the cheque.
Examples from the Corpus
cosignatory• We will need both cosignatories to sign the check. 