codice:
<script type="text/javascript">
/*<![CDATA[*/
function fillSelect(idSelect,from){
	var sel=document.getElementById(idSelect);
	sel.options.length=0;
	for(var k=from;k<=100;++k){
		sel.options[sel.options.length]=new Option(k,k);
	}
}
// quello che segue a te non serve
window.onload=function(){
	fillSelect('select1',1);
	fillSelect('select2',1);
}
/*]]>*/
</script>

<form>
<select id="select1" onchange="fillSelect('select2',parseInt(this.options[this.selectedIndex].value)+1)">
</select>
<select id="select2">
</select>
</form>
ciao