co-sign

A signature in addition to the principal signature of another in order to verify the authenticity of the principal signature. 163 A. 787, 788. The term does not have a definite legal significance as to whether the signature is one of a principal or a surety. 121 Cal. Rptr. 453, 455. However, an indorsement which clearly shows that it is not in the chain of title is notice that it is there only as an accommodation. U.C.C. §3-415(4).

The verb has 2 meanings:

Meaning #1: sign jointly
  Synonym: cosign

Meaning #2: sign and endorse (another person's signature), as for a loan
  Synonym: cosign