Is the structuring done by designating two beneficiaries?
https://www.fdic.gov/resources/deposit-insurance/trust-accounts/
Generally, it’s ownership of the account. Being an authorized signatory doesn’t prove ownership.
So Husband - 250,000
Wife - 250,000
Husband and Wife - $250.000
Different rules for beneficiaries under at trust, which I don’t recall at this point, except I think there are different rules for non-profits and whether they are domestic or foreign.
If the bank fails, the FDIC is going off of the official paperwork for the account, and doesn’t care what you and the bank intended to do. And generally banks that fail aren’t known for perfect paperwork.