Hi,
so gehts:
<script language="javascript">
function OpenMyLink(URL1,URL2,TARGET1,TARGET2){
parent.frames[TARGET1].location.href=URL1;
parent.frames[TARGET2].location.href=URL2;
}
</script>
<a href="OpenMyLink('seite1.html','seite2.html','mainFrame','leftFrame')">Klick mich an!</a>