Can I be sued and made liable for my husband’s hit and run because I am the co-owner of the vehicle and “allowed” him to drive knowing his problems?

You don't mention what problems your husband had, but generally, only the person that committed the hit and run can be sued or prosecuted for the hit and run.  You would not have criminal liability for this offense.  However, the fact that your name is on the title does create some potential for civil liability.  You wil have a defense being that he was a co-owner and had as much right to the vehicle as you did, and therefore could not have legally been able to stop him.  The exception would be if you were some type of guardian and you were responsible for care, such that some type of liability for neglect of duties could be imposed.  What's done is done already, but you may want to make notes on what happened that morning while your memory is still fresh that sets out what you did to prevent or object to his taking of the vehicle.  If you wait for six months or longer, you could forget important details.  If anyone heard any part of the conversation, you should have them do the same and makes notes about what you believe they heard and saw.  Going forward, you may want to make arrangements divide ownership and use of the vehicles-- and you need to keep the keys to your vehicle in a secure place to prevent a repeat event.