You can show your popup using javascript. Try this:
Javascript:
function fnShowPopup(sender) {
var popup = document.getElementById('ModelPopup1');
if (sender.id == "Button1") {
popup.show();
}
else {
popup.show();
}
}
HTML:
<asp:button id="Button1" runat="server" autopostback="false" onclick="fnShowPopup(this)" xmlns:asp="#unknown" />
<asp:button id="Button2" runat="server" autopostback="false" onclick="fnShowPopup(this)" xmlns:asp="#unknown" />
--Amit