function popUp() {
  if((document.getElementById("chN1").value == "null" && document.getElementById("chS1").value == "null") || (document.getElementById("chN2").value == "null" && document.getElementById("chS2").value == "null"))
  {
    alert("Bitte beide Elemente auswählen.");
    return;
  }

  var element1, element2;

  if(document.getElementById("chN1").value != "null") { element1 = document.getElementById("chN1").value; }
  else { element1 = document.getElementById("chS1").value; }

  if(document.getElementById("chN2").value != "null") { element2 = document.getElementById("chN2").value; }
  else { element2 = document.getElementById("chS2").value; }

  var fenster = open("tab11.php?element1=" + element1 + "&element2=" + element2, "popup", "width=400, height=385");
}

function check(obj)
{
  if(obj.value != "null")
  {
    if(obj.name == "chN1") { document.getElementById("chS1").value = "null"; }
    if(obj.name == "chS1") { document.getElementById("chN1").value = "null"; }

    if(obj.name == "chN2") { document.getElementById("chS2").value = "null"; }
    if(obj.name == "chS2") { document.getElementById("chN2").value = "null"; }
  }
}