Quello che intendo è questo:

codice:
<script type="text/javascript" defer="defer">
$(document).ready(
    function(){
        $('#andata').datepicker(
            {
            changeMonth: true,
            showButtonPanel: false,
            minDate: '04/01/2014',
            maxDate: '10/31/2014',
            onSelect: function()
                {
                var date = $('#andata').datepicker({ dateFormat: 'mm/gg/yyyy' }).val();
                $('#data_a').val(date);
                var day = $('#andata').datepicker('getDate');
                var day2 = $('#andata').datepicker('getDate');
               if ((day2.getDate()-day.getDate())<7) day.setDate(day.getDate()+7);
                $('#ritorno').datepicker('option', 'minDate', day );
                var date2 = $('#ritorno').datepicker({ dateFormat: 'mm/gg/yyyy' }).val();
                $('#data_b').val(date2);
                }
            }
        );
        $('#ritorno').datepicker(
            {
            minDate: '04/07/2014',
            maxDate: '10/31/2014',
            changeMonth: true,
            showButtonPanel: false,
            onSelect: function()
                {
                var date2 = $('#ritorno').datepicker({ dateFormat: 'mm/gg/yyyy' }).val();
                $('#data_b').val(date2);
                }
            }
        );
    }
);
</script>