link two datepickers with jquery

I have two datepickers one of them is for a start date and the other is for an end date. My problem is when i select a start date ,the end date datepicker should be after this date so that i should lock the other dates and if I select an end date input, the start date input should be after the end date.

HTML:

<div class="row">
    <div class="col-sm-5">
        <div class="form-group">
            <label class="control-label col-sm-5" for="date_added">Date
                Courrier</label>
            <div class="input-group date">
                <span class="input-group-addon"><i
                    class="fa fa-calendar"></i></span><input id="date_added"
                    type="text" class="form-control" value="03/04/2014">
            </div>
        </div>
    </div>
    <div class="col-sm-5">
        <div class="form-group">
            <label class="control-label col-sm-7" for="date_modified">Date
                Arrivée</label>
            <div class="input-group date">
                <span class="input-group-addon"><i
                    class="fa fa-calendar"></i></span><input id="date_modified"
                    type="text" class="form-control" value="03/06/2014">
            </div>
        </div>
    </div>    
</div>

Javascript:

<script>

$(document).ready(function() {
    var start_date=$('#date_added').val();
    var end_date=$('#date_modified').val();


    $('#date_added').datepicker({

        onSelect: function(dateStr) {

            start_date=$(this).val();
            $('#date_modified').datepicker('option', 'minDate', new Date(start_date));

        },
        todayBtn : "linked",
        keyboardNavigation : false,
        forceParse : false,
        calendarWeeks : true,
        autoclose : true
    });
    console.log("jjj")
    $('#date_modified').datepicker({
        onSelect: function(dateStr) {
            alert("Date is within two weeks");

            end_date=$(this).val();
            $('#date_added').datepicker('option', 'minDate', new Date(end_date));

        },
        todayBtn : "linked",
        keyboardNavigation : false,
        forceParse : false,
        calendarWeeks : true,
        autoclose : true
    });

});


</script>

So what should I change