coinsurer

[ koh-in-shoo r-er, -shur- ]
/ ˌkoʊ ɪnˈʃʊər ər, -ˈʃɜr- /

noun

a person or firm that contracts as an insurer jointly with another or others.
an insured who is liable to bear part of the loss under a coinsurance provision.

Origin of coinsurer