This works:

Code:
<HTML>
<HEAD>
<script language=javascript>
function doit(x)
{
	if(x==0)
	{
		document.frm1.s2.selectedIndex=0;
	}
}
</script>

</HEAD>
<BODY>

<form name="frm1">
<select name="s1" onclick="doit(this.selectedIndex)">
	<option value=0>No</option>
	<option value=1>Yes</option>
</select>
<select name="s2">
	<option value=0>No</option>
	<option value=1>Yes</option>
</select>
</form>

</BODY>
</HTML>