I don’t really understand your code, do you have 2 forms on the page?
Anyway, to create a new user you need to use a different “post_id” something like “new_user” as your post ID.
Then you need to use an acf/pre_save_post filter that checks for your “post_id” value, inserts the new user and then returns the correct user id in the format "user_{$new_user_id}"