Can you unilaterally sell a car if it is titled as joint owners?

That would depend upon state laws and how the car is titled. Generally if a vehicle is titled "John Smith and whomever" then the vehicle can only be sold if both agree or by order of the court. If the title reads "John Smith or whomever" the car can be sold by either person. This would not apply in some instances, some examples...pending divorce proceedings, legal action by creditors, or probate procedures, etc.