<div ng-controller="MainController">
      <div ng-controller="SecondController">{{number}}</div>
    </div>

$scope.number = Number.get ();

<div ng-controller="MainController as main">
   <div ng-controller="SecondController as second">{{main.number}}</div>
</div>

<div ng-controller="MainController">
  <div ng-controller="SecondController">{{$parent.number}}</div>
</div>