cosigner
1. [ noun ] one of two or more signers of the same document (as a treaty or declaration)
Synonyms: cosignatory
Related terms: signer co-sign
2. [ noun ] a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)
Synonyms: cosignatory
Related terms: signer co-sign
Similar spelling:   cosign
  co-sign
  cockaigne
  cosignatory