You can actually deep dive into directive controllers and "transclusions".
To access parent controller you can use require option.
.directive 'parent', ->
  controller: ->
    @addHeader = (header) => #do add header
.directive 'child', ->
  require: '^parent'
  link: (scope, el, attr, parent) ->
    parent.addHeader 'from child'
But you need to make sure your child link function actually ran. 
For example (WARNING JAVASCRIPT!!! :) you can use transclude option. Sophisticated Example.
  .directive('myTable', function() {
    return {
      restrict: 'E',
      controller: function() {
        var headers = []
        this.headers = headers
        this.addHeader = headers.push.bind(headers)
      },
      template: `
        <table>
          <thead>
            <tr>
            </tr>
          </thead>
        </table>
      `,
      transclude: {
        // transclude all myHeaders into headers slot
        headers: 'myHeader' // transclude (how this is a real word at all?)
      },
      link: function(scope, el, attrs, ctrl, transclude) {
        var headerRow = el.find('thead').children('tr')
        // append all headers into thead wrapping with th
        transclude(function(headers) {
          [].forEach.call(headers, header => {
            var cell = angular.element('<th></th>')
            cell.append(header)
            headerRow.append(cell)
          })
        }, headerRow, 'headers')
        console.log(ctrl.headers) // headers were populated here
      }
    }
  })
  .directive('myHeader', function() {
    return {
      restrict: 'E',
      require: '^myTable',
      transclude: true, // ohh more transclusions
      template: '<span ng-transclude></span>', 
      link: function(scope, el, attrs, myTable) {
        myTable.addHeader(attrs.name) // report to myTable
      }
    }
  })
<my-table>
  <my-header name="First"> First Header </my-header>
  <my-header name="Second"> Second <span style="color:red;">Header</span> </my-header>
</my-table>