Try below code:
XmlDocument objXml = new XmlDocument();
objXml.Load("testXML.xml");
XmlNodeList objFirst = objXml.SelectNodes("main/first");
foreach (XmlNode xNode in objFirst)
{
foreach (XmlNode xSecondNode in xNode.SelectNodes("second"))
{
Response.Write(xSecondNode.Attributes[0].Value.ToString() + "<br/>");
}
}