How to bind 2 models to one input field in Angular?

You cannot, but there are some workarounds.

1. Use ngChange to update the other model

<input type="text" 
       ng-model="sn_number" 
       ng-change="id=sn_number"/> 

2. You could watch a model, and when in changes, update another

$scope.$watch('sn_number', function(v){
  $scope.id = v;
});

You would need to watch also for changes in id if you want to keep them in sync.

Example here

You can bind fields immediately, not only in ng-change, and actually it isn't data binding, its only angular expression

  <label>Name</label>
  <input type="text" ng-model="name" value="{{name}}"/>

  <label>Key</label>
  <input type="text" ng-model="key" value="{{key=name}}" />

It would make no sense to bind an input to two variables in the model. Binding works both ways, so if the model is updated, the field is updated and vice-versa. If you were to bind to two variables, what would be the single source of truth?

However you can use ng-change to call a method on the controller which can set two variables when the field changes.