www.linguee.com.br/ingles-portugues/traducao/is+signer+of+the+.html‎
Em 1989, Küng foi co-assinante da auto denominada 'Declaração de Colónia', um voto para um catolicismo receptivo e contra uma maior ampliação da ...

Meaning of junto in Portuguese english dictionary - Significado de ...
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viver com , morar junto com. co: co - ( junto , em conjunto ). co - signer: assinante comum ( pessoa que assina um compromisso junto com outros ). coactivated:.

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Note added at 13 minutos (2013-12-04 13:42:29 GMT)
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PT-BR

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Note added at 16 minutos (2013-12-04 13:46:02 GMT)
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co-signer
assinante comum (pessoa que assina um compromisso junto com outros)

http://thesaurus.babylon.com/co-signer

Aqui no contexto de emprestimos, entendo que estão se referindo a fiador/avalista.

http://www.businessdictionary.com/definition/co-sign.html
To sign and endorse jointly. Someone who co-signs a loan will be responsible for paying back the debt if the other individuals fails to make their payments. "Steve co-signed for the loan so that Amy was able to obtain a car loan despite her poor credit history."


http://en.wikipedia.org/wiki/Co-signing
Co-signing often involves a promise to pay another person's debt arising out of contract if that person fails to do so. Conversely, co-signing may involve taking on a joint, or joint-and-several, obligation to pay a debt, regardless of whether or not the other co-signer fails to do so. In the first instance the co-signer's obligation may be in the nature of a guaranty, while in the second instance the co-signer's obligation might be described as that of a co-debtor.

***A co-signer is a person who is the guarantor of the contract, especially a promissory note. A co-signer is also known as a surety.***

MICHAELIS:
fi.a.dor
sm 1 guarantor, surety, sponsor, guarantee. 2 guaranty, bond(sman), security, surety. 3 rifle prop or rifle rest. 4 rein, strap of a bridle. 5 sword knot. 6 Náut mooring bridle. fiador de seda (in spinning) thrower. ser fiador to stand surety.