The easiest way to accomplish this is to have a radio field that allows selecting what type of value to enter with both of the select fields being conditional on the radio field. Then in you template you get the value of the radio field and display the correct content.
If you want to remove the content of the other field that is hidden you need to create an acf/update_value filter https://www.advancedcustomfields.com/resources/acf-update_value/. This will only run when a field is being updated, when one field is update you delete the value from the other field https://www.advancedcustomfields.com/resources/delete_field/
