You could combine all in one array and then start to encode.
$dataSet = array();
$data1='<p>hehe</p>';
$data2='<h1>wawa</h1>'; 
$dataSet[] = $backarr = array($data1,$data2);
$sql="SELECT username FROM users WHERE username != '$username'"; 
$result = mysql_query($sql) or die(mysql_error()); 
while($row = mysql_fetch_array( $result )) { 
    $names[] = $row['username'];
} 
$dataSet[] = $names;
echo json_encode($dataSet);
EDIT::
And here the jQuery stuff:
jQuery.post('url/script.php',{fieldName : fieldValue},function(res){
   console.log(res);
   var res = jQuery.parseJSON(res);
   console.log(res);
});
Pure AJAX:
<script type="application/javascript">
function loadJSON()
{
   var data_file = "http://www.example.com/data.json";
   var http_request = new XMLHttpRequest();
   try{
      // Opera 8.0+, Firefox, Chrome, Safari
      http_request = new XMLHttpRequest();
   }catch (e){
      // Internet Explorer Browsers
      try{
         http_request = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
         try{
            http_request = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
            // Something went wrong
            alert("Your browser broke!");
            return false;
         }
      }
   }
   http_request.onreadystatechange  = function(){
      if (http_request.readyState == 4  )
      {
        // Javascript function JSON.parse to parse JSON data
        var jsonObj = JSON.parse(http_request.responseText);
        // jsonObj variable now contains the data structure and can
        // be accessed as jsonObj.name and jsonObj.country.
        document.getElementById("Name").innerHTML =  jsonObj.name;
        document.getElementById("Country").innerHTML = jsonObj.country;
      }
   }
   http_request.open("GET", data_file, true);
   http_request.send();
}
</script>