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
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