Untitled

<!DOCTYPE html>
<html>
<body>

<select id="select" size="8">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
</select>

<button onclick="levo();">Levo</button>
<button onclick="desno();">Desno</button>

<select id="select2" size="8">
  <option value="audi">Audi</option>
</select>

<script>
function desno() {
  var select = document.getElementById("select");
  var select2 = document.getElementById("select2");
  var option = document.createElement("option");
  var index = select.selectedIndex;
  option.text=select.options[index].innerHTML;
  select.remove(index);
  select2.add(option);
}

function levo() {
  var select = document.getElementById("select");
  var select2 = document.getElementById("select2");
  var option = document.createElement("option");
  var index = select2.selectedIndex;
  option.text=select2.options[index].innerHTML;
  select2.remove(index);
  select.add(option);
}
</script>
</body>
</html>