what do i do if my car is in both our name, and we are separating

If you want a co owner to transfer title, you can simply ask her.  If she refuses, that is something that could be resolved in family court during the divorce.  However, it is neither inexpensive nor quick.  There are only 2 ways to resolve property issues in a divorce, agreement or litigation.  The latter can take many months and sometimes years while costing soemtimes thousands of dollars.  I always encourage agreements if they can be achieved on reasonable terms.