Esto se hace con javascript.
Aquí tienes el código:
<!-- PRIMER PASO: Copiar dentro del tag HEAD -->
<script LANGUAGE="JavaScript">
<!--
sortitems = 1;
function move(fbox,tbox) {
for(var i=0; i<fbox.options.length; i++) {
if(fbox.options[i].selected && fbox.options[i].value != "") {
var no = new Option();
no.value = fbox.options[i].value;
no.text = fbox.options[i].text;
tbox.options[tbox.options.length] = no;
fbox.options[i].value = "";
fbox.options[i].text = "";
}
}
BumpUp(fbox);
if (sortitems) SortD(tbox);
}
function BumpUp(box) {
for(var i=0; i<box.options.length; i++) {
if(box.options[i].value == "") {
for(var j=i; j<box.options.length-1; j++) {
box.options[j].value = box.options[j+1].value;
box.options[j].text = box.options[j+1].text;
}
var ln = i;
break;
}
}
if(ln < box.options.length) {
box.options.length -= 1;
BumpUp(box);
}
}
function SortD(box) {
var temp_opts = new Array();
var temp = new Object();
for(var i=0; i<box.options.length; i++) {
temp_opts[i] = box.options[i];
}
for(var x=0; x<temp_opts.length-1; x++) {
for(var y=(x+1); y<temp_opts.length; y++) {
if(temp_opts[x].text > temp_opts[y].text) {
temp = temp_opts[x].text;
temp_opts[x].text = temp_opts[y].text;
temp_opts[y].text = temp;
temp = temp_opts[x].value;
temp_opts[x].value = temp_opts[y].value;
temp_opts[y].value = temp;
}
}
}
for(var i=0; i<box.options.length; i++) {
box.options[i].value = temp_opts[i].value;
box.options[i].text = temp_opts[i].text;
}
}
// -->
</script>
<!-- SEGUNDO PASO: Copiar dentro del tag BODY -->
<form ACTION METHOD="POST">
<table border="0">
<tr>
<td><select multiple size="5" name="list1">
<option value="11"> item 1.1 </option>
<option value="12"> item 1.2 </option>
<option value="13"> item 1.3 </option>
</select></td>
<td><input type="button" value=" >> "
onclick="move(this.form.list1,this.form.list2)" name="B1"><br>
<input type="button" value=" << "
onclick="move(this.form.list2,this.form.list1)" name="B2"> </td>
<td><select multiple size="5" name="list2">
<option value="21"> item 2.1 </option>
<option value="22"> item 2.2 </option>
<option value="23"> item 2.3 </option>
</select></td>
</tr>
</table>
</form>