You need to put two
RequiredFieldValidator with corresponding
ValidationGroup. Try like this:
<asp:textbox id="txt" runat="server"></asp:textbox>
<asp:requiredfieldvalidator runat="server" id="rfv1" controltovalidate="txt" validationgroup="VG1" errormessage="Validation Faild with button 1"></asp:requiredfieldvalidator>
<asp:requiredfieldvalidator runat="server" id="rfv2" controltovalidate="txt" validationgroup="VG2" errormessage="Validation Faild with button 2"></asp:requiredfieldvalidator>
<asp:button id="btn1" runat="server" text="Button 1" validationgroup="VG1" />
<asp:button id="btn2" runat="server" text="Button 2" validationgroup="VG2"/>
--Amit