Filter results from dropdown

<script language="JavaScript">
function nav()
   {
   var w = document.change.goto.selectedIndex;
   var url_add = document.change.goto.options[w].value;
   window.location.href = url_add;
   }
function nav2()
   {
   var w2 = document.change.goto2.selectedIndex;
   var url_add2 = document.change.goto2.options[w2].value;
   window.location.href = url_add2;
   }
</script>
<form action="[~[*id*]~]" method="get" name="change" id="change">
Show <select name="goto" onChange="nav();" style="width:110px;">
<option selected>sort 1</option>
<option value="[~[*id*]~]&tags=tag1 name">tag1 name</option>
<option value="[~[*id*]~]&tags=tag2">tag2</option>
<option value="[~[*id*]~]&tags=tag3">tag3</option>
<option value="[~[*id*]~]&tags=tag4 name">tag4 name</option>
</select>
<select name="goto2" onChange="nav2();" style="width:110px;">
<option selected>sort 2</option>
<option value="[~[*id*]~]&tags=tag1/name">tag1/name</option>
<option value="[~[*id*]~]&tags=tag2/name">tag2/name</option>
<option value="[~[*id*]~]&tags=tag3/name">tag3/name</option>
<option value="[~[*id*]~]&tags=tag4/name">tag4/name</option>
<option value="[~[*id*]~]&tags=tag5">tag5</option>
</select>
</form>

[[Ditto? &id=`catal` &startID=`[*id*]` &orderBy=`menuindex ASC` &display=`[[how_many]]` &tpl=`ditto_tpl` &tplAlt=`ditto_alttpl` &paginate=`1` &paginateAlwaysShowLinks=`0` &tagData=`tvname1,tvname2` &tagDelimiter=`,` &tagDisplayMode=`1` &tagSort=`0` &tagMode=`onlyTags` &tags=`[!get.tvname!]` &depth=`10`]]

return $_GET['tvname1'];
return $_GET['tvname2'];

<option value="[~[*id*]~]&tags=tag2">tag2</option>

<option value="[~[*id*]~]&tags=catal_tag2">tag2</option>