I don’t know of the OP found a solution. This question was asked a long time ago.
I would use select fields and not taxonomy fields. For the first field I would dynamically generate the values of the “parent” category form the information here https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/
For the child category I would use ajax to populate the select field. I have created an example of doing this here https://github.com/Hube2/acf-dynamic-ajax-select-example/tree/master/dynamic-select-example. This example uses posts in a post type, but the principle is the same.
You would also need to create an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/ if you want to update the post categories https://codex.wordpress.org/Function_Reference/wp_set_post_terms since you wouldn’t be using a taxonomy field that would do this automatically.
