Look at the UTMA in your state (your profile doesn't indicate which one it is). As noted, some specifically say that only one custodian is permitted.
I suggest you comply with the law, not the check. If your state only allows one custodian, the issue belongs to the insurance company that cut the check in this fashion.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.