It cannot be done this way using save and load terms on ACF fields. You will need to create an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/, get the values from the two fields and use wp_set_object_terms() https://codex.wordpress.org/Function_Reference/wp_set_object_terms to update them yourself.
ACF uses false for $append, if it did not then it could never remove a term when one is deleted from the field.
