As Brian said, if you are using a dropdown, if you listed all the states, you wouldn't need a field for the user to enter the value.
If you wanted to use a user specified field, you could use a radio button group to specify which value to use when the form is submitted. When processing the form, you will have to add some code to check the radio button that is selected and use the corresponding value. Also, you would most likely need to not validate the two fields or add additional code to determine which needs to be validated before validating the field.