how to assing 2 validation groups to a button.

I thought I could assign more than 2 validation groups, separated by comma as follows.

<asp:Button ID="Button2" runat="server" Text="both v1 & v2 validate" ValidationGroup="v1, v2" onclick="Button2_Click"/>
but it didn't work.
What i want to do is, when a user clicks button1, it only validates validation group"v1"
When button2 is clicked, it validates both "v1" & "v2" validatino groups.

<asp:TextBox ID="TextBox1" runat="server" ValidationGroup="v1"/>
        <asp:RequiredFieldValidator
            ID="RequiredFieldValidator1" runat="server" ErrorMessage="*" ValidationGroup="v1" ControlToValidate="TextBox1"/>
      &n ...