Here's one option, use javascript's onkeyup event on the client side to clear the result. When the user selects the val1 control and presses and releases a key the result value will be set to an empty string.
<script type="text/javascript">
function ResetResultValue() {
var el = document.getElementById('<%= res.ClientID %>');
el.value = '';
}
</script>
<asp:textbox id="val1" runat="server" onkeyup="javascript:ResetResultValue();"></asp:textbox>
<asp:textbox id="val2" runat="server"></asp:textbox>
<asp:textbox id="res" runat="server"></asp:textbox>
<asp:button id="btnSubmit" runat="server" text="Submit" onclick="btnSubmit_Click" />