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.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.