javascript validation for when we don't want to select both values and when we want to select both

<script>
function fun(){
var empt = document.forms["getCondDetails"]["formuserid"].value;
var Classname = document.forms["getCondDetails"]["Classname"].value;
  if (empt == "" && Classname == "")
  {
  alert("Please input a Values");
  return false;
  }
  if (empt != "" && Classname != "")
  {
  alert("Please don't submit both ID number and remaining fields");
  return false;
  }

 return true;

}
</script>