I don’t have an exact answer for you, only a possible path to solving it.
The first thing you’ll need to to is to alter the ajax data being sent for each field to include the values selected in the other field.
Then in the query filters for each field you need to get the values sent in the ajax query and use that value in the ‘exclude’ argument for get_terms().
Sorry that I cannot provide any code for you.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!