this is how i use multiple field as textfield in a single combobox. It works for me :) but Please CMIIW... ;)
dtbase.php :
$rs = mysql_query("select id_customer,concat(firstname_customer,' ',lastname_customer) AS name
FROM customer order by firstname_customer ASC");
$items = array();
while($row = mysql_fetch_object($rs)){
array_push($items, $row);
}
echo json_encode($items);
-------
<input name="id" style="width:300px" class="easyui-combobox" url="dtbase.php"
valueField="id_customer" textField="name" required="true">