var toggleSelectBox = function(val, idarr, selectBox) {
    if (idarr[val])
    {
        selectBox.val('');
        selectBox.attr("disabled", "true");
    }
    else
        selectBox.removeAttr("disabled");
};

$(document).ready(function() {
    $('#selectedSkaterId').change(function() {
        var val = $(this).val();
        var sess1sel = $('#selectedFirstSessionClassId');
        var sess2sel = $('#selectedSecondSessionClassId');

        toggleSelectBox(val, sess1, sess1sel);
        toggleSelectBox(val, sess2, sess2sel);
    });
});