co-signatory

co-signatory
cosignatory, / ˌkəuˈsɪgnətərɪ; ?@ -tɔ:rɪ; ko`sɪɡnəˌtɔrɪ/ n ~ (of/to sth) person, state, etc signing jointly with others 连署者（人、 国家等）
    The US and the former Soviet Union were co-signatories of/to the treaty. 美国和前苏联都是条约的签署国.

牛津英汉双解词典