Empty ng-model overwrites ng-value - angularjs

Question

when I use ng-model together ng-value. ng-value returns empty even there is a value given.
for example, there are two controllers called FirstCtrl and SecondCtrl.
This is FirstCtrl, and If I type a value for this input, it will write in my SeconCtrl input since I call ng-value="firstname" in my SecondController:
I can pass the value from firstctrl to secondctrl(I mean first input field to second input field). But the second input field has an ng-model called d_firstname, which doesn't allow to save ng-value but ng-value is displaying the correct value but saves empty because of ng-model(d_firstname). However, i need d_firstname if user wants to enter new value. Moreover, If I type over the input field in the secondCtrl, I can save the record because of d_firstname but if I keep not modifying anything in that input field, returns empty. hope you get the problem. Please someone help on this.. I wanted to pass data from one form to other if checkbox clicked for billing details and shopping details are same
FirstController input:
<input type="text" name="firstname" ng-model="firstname" required>
SecondCtrl input:
<input name="firstname" ng-value="firstname" ng-model="d_firstname">

Answer 1

There is no point in using ng-value and ng-model together.
You could use ng-change event to update the second one
<input name="firstname" ng-model="firstname" ng-change="updateName()" required>
<input name="d_firstname" ng-model="d_firstname">
Controller:
$scope.updateName = function(){
$scope.d_firstname = $scope.firstname
}
Note that you should always use an object in ng-model. You will run into problems with child scopes if you don't