var files = ['john.xml','doe.xml']
var newResult, temp;

function loadMyXMLFiles(){

    // > let's load file by file using jquery's ajax-call:

    $.each(files, function(index,value) {

      $.ajax({

          url: value,
          cache: true,
          success: function(data){

            // > check if temp is empty, append or fill

            (temp) ? temp += (data) : temp = data;


            // > on complete:

            if (index == files.length-1) {

                xmlResult = temp; temp = null;
            }
          }

      });
  });
}

loadMyXMLFiles()

var files = ['john.xml','doe.xml']
var xmlResult, temp = ""; //Make temp an empty string

loadMyXMLFiles(){
    $.each(files, function(index,value) {
      $.ajax({
          url: value,
          cache: true,
          async: false, //This is crucial!
          success: function(data){
            temp += data;
            if (index == files.length-1) {
                xmlResult = temp; temp = null;
            }
          }
      });
  });
}