hi,
just place an asp panel inside uraspx page..and make that the default button name as ur second bitton id..here am giving you one example..
Hope this may help you..
<asp:panel id="Panel1" runat="server" defaultbutton="btn2" xmlns:asp="#unknown">
<asp:button id="btn1" runat="&"server"/">
<asp:button id="btn2" runat="server" />