<iframe id="ti" width="100%" frameborder="0" src="Webform1.aspx" height="100"></iframe>
<br>
<select name="ListBox1" size="10" id="ListBox1" onchange="ti.location.href=\"+this.value+'aspx'" style="width:184px;">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="5">5</option>
</select>
<br>
<select name="ListBox1" size="10" id="ListBox1" onchange="ti.location.href=\"+this.value+'aspx'" style="width:184px;">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="5">5</option>
</select>
{
document.all.ti.src ="1.aspx";
}