You can create a filter for this.
add_filter('acf/fields/user/query/name=your_field_name', 'set_user_blog_id', 10, 3);
function set_user_blog_id($args, $field, $post_id) {
// set blog id in args
$args['blog_id'] = 2;
return $args;
}
I do not believe that get_users() allows you to get users for more than one blog id from what I can see reading the doc https://codex.wordpress.org/Function_Reference/get_users.
