使用.first()and .last()，因为您选择了多个元素

//for moving elements up and down
$('.up-button').click(function(){
 before = $('#select2 option:selected').first().prev();
    $('#select2 option:selected').insertBefore(before);

});

$('.down-button').click(function(){
   after = $('#select2 option:selected').last().next();
    $('#select2 option:selected').insertAfter(after);
}); 

稍微改变了向上/向下逻辑：

$('.up-button').click(function(){
  $('#select2 option:selected:first-child').prop("selected", false);
  before = $('#select2 option:selected:first').prev();
    $('#select2 option:selected').detach().insertBefore(before);

});

$('.down-button').click(function(){
  $('#select2 option:selected:last-child').prop("selected", false);
  after = $('#select2 option:selected:last').next();
    $('#select2 option:selected').detach().insertAfter(after);
}); 

小提琴