AngularJS: How can I pass variables between controllers?

I have two Angular controllers:

function Ctrl1($scope) {
    $scope.prop1 = "First";
}

function Ctrl2($scope) {
    $scope.prop2 = "Second";
    $scope.both = Ctrl1.prop1 + $scope.prop2; //This is what I would like to do ideally
}

I can’t use Ctrl1 inside Ctrl2 because it is undefined. However if I try to pass it in like so…

function Ctrl2($scope, Ctrl1) {
    $scope.prop2 = "Second";
    $scope.both = Ctrl1.prop1 + $scope.prop2; //This is what I would like to do ideally
}

I get an error. Does anyone know how to do this?

Doing

Ctrl2.prototype = new Ctrl1();

Also fails.

NOTE: These controllers are not nested inside each other.