Hi @bobz
Thanks for the question.
At the moment this kind of event driven select field is not yet possible with the core of ACF.
It is something I will look at in the near future, but for now you will need to write some custom AJAX to listen to a change event on the first select and then use PHP / AJAX to return the possible choices for the second select.
You can use jQuery to then populate the second select form with these choices.
Hope that helps.
Thanks
E
