You would need to create custom JavaScript for the field in question, make an AJAX request and do your own query for the select2 field of the taxonomy field. This isn’t something that you can to with ACF. You can start here: http://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
Another way you could do this is to use a regular select field that you dynamically populate from the first taxonomy then create a conditional select fields that show/hide based on the selection of the first select field. A bit complicated to build.
