You can access the scope on which the event was fired with event.targetScope
$scope.$on('ngGridEventSorted', function(event, args) {
var targetScope = event.targetScope;
// inspect targetScope's properties to differentiate between the two grids
});
Another way would be to create two wrapping div's around the grids, each with their own controller that handles the event.
<div ng-controller="controllerOne">
<ng-grid ...>
</div>
<div ng-controller="controllerTwo">
<ng-grid ...>
</div>