I have two validation summary controls on one page. Both have different validation group. please look below code.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm11.aspx.cs" Inherits="TestApp.WebForm11" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <asp:ValidationSummary ID="ValidationSummary1" runat="server" ValidationGroup="g1"/>
            <asp:Label ID="Label1" runat="server" Text="label1"></asp:Label>
            <asp:TextBox ID="TextBox1" runat="server" Text="A"></asp:TextBox>
            <asp:CustomValidator ID="CustomValidator1" runat="server" ValidationGroup="g1" OnServerValidate="CustomValidator1_ServerValidate">*</asp:CustomValidator>
            <br />
            <asp:Button ID="Button1" runat="server" Text="Button" ValidationGroup="g1" CausesValidation="false" OnClick="Button1_Click" />
            <br />
            <asp:ValidationSummary ID="ValidationSummary2" runat="server" ValidationGroup="g2"/>
            <asp:Label ID="Label2" runat="server" Text="label1"></asp:Label>
            <asp:TextBox ID="TextBox2" runat="server" Text="B"></asp:TextBox>
            <asp:CustomValidator ID="CustomValidator2" runat="server" ValidationGroup="g2" OnServerValidate="CustomValidator2_ServerValidate">*</asp:CustomValidator>
            <br />
            <asp:Button ID="Button2" runat="server" Text="Button" ValidationGroup="g2" CausesValidation="false" OnClick="Button2_Click" />
            <br />
        </div>
    </form>
</body>
</html>
code behind
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace TestApp
{
    public partial class WebForm11 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }
        protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
        {
            args.IsValid = true;
            int num;
            if(!(int.TryParse(TextBox1.Text, out num)))
            {
                CustomValidator1.ErrorMessage = "cv1 msg";
                args.IsValid = false;
            }
        }
        protected void CustomValidator2_ServerValidate(object source, ServerValidateEventArgs args)
        {
            args.IsValid = true;
            int num;
            if (!(int.TryParse(TextBox2.Text, out num)))
            {
                CustomValidator2.ErrorMessage = "cv2 msg";
                args.IsValid = false;
            }
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            Page.Validate("g1");
            if(this.Page.IsValid)
                Response.Write("button1 success");
            else
                Response.Write("button1 falied");
        }
        protected void Button2_Click(object sender, EventArgs e)
        {
            Page.Validate("g2");
            if (this.Page.IsValid)
                Response.Write("button2 success");
            else
                Response.Write("button2 falied");
        }
    }
}
Now, when I click on button1, it validate the validation group "g1" and show
 
  
And when click on button2, it validate the validation group "g2" and show

hmmmmmmm, but I don't want to loose validation summary 1 (group g1) message on click on button2.
Also, I don't want to validate both groups g1 and g2 on click event.
Why I am loosing validation g1 message on button2 click event.
 
                        
It is because only one validation happens in a either click event. You may want to check both validations in both click events. Or insert both on Page reload event.