co ‧ sig ‧ na ‧ to ‧ ry /ˌkəʊˈsɪɡnət ə ri $ ˌkoʊˈsɪɡnətɔːri/ BrE AmE noun ( plural cosignatories ) [countable]
one of a group of people who sign a legal document for their department, organization, country etc:
Both cosignatories must sign the cheque.