Sign jointly.

synonym: co-sign.

Roget 771: give security, give bail, give substantial bail; go bail; pawn, impawn^†, spout, mortgage, hypothecate, impignorate^†.    guarantee, warrant, warrantee, assure; accept, indorse, underwrite, insure; cosign, countersign, sponsor, cosponsor.    execute, stamp; sign, seal etc. (evidence) 467.    let, sett^†; grant a lease, take a lease, hold a lease; hold in pledge; lend on security etc. 787.    ... show more

Sign and endorse (another person's signature), as for a loan.

synonym: co-sign.