To coinsure is an insurance-office word about splitting the cost of risk. It can work two ways. One: two insurers share one policy, so each pays part if something goes wrong. Two: the cover itself will only pay a share of the loss, and you pay the rest. That second idea is common in health and property insurance: the company might coinsure 80 percent of a hospital bill, and you pay 20 percent. Ordinary people rarely say the verb out loud; they talk about coinsurance, or they say the company pays part and I pay part. This is technical English, not everyday chat.

共同承保；共同投保（保險業常說「共保」）