I think I know that AB also implements IRequiresSessionState, without blocking the interface. It seems that Session has an impact on concurrency, so you should use it with caution.

Session implements read-write locks in

asp.net.
your problem is that write locks are implemented in a.aspx, blocking access to session for b.aspx pages.
if you want to access it at the same time, set the a.aspx as follows
<% @ Page EnableSessionState= "ReadOnly"% >, as well as in b.aspx, because read locks can block write locks.
now b.aspx can be accessed normally.
similarly, if you want to customize the implementation of IHttpHandler, you need to implement IRequiresSessionState and IReadOnlySessionState two tag interfaces.
from MSDN
https://social.msdn.microsoft.

//
//CPU
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace 
{
    /// <summary>
    /// :
    /// 1.BeginEnvoke EndEnvoke
    /// BeginInvokeEndInvoke
    /// </summary>
    class Program
    {
        public delegate void PrintDelegate(string s);
        static void Main(string[] args)
        {
            PrintDelegate printDelegate = Print;
            Console.WriteLine("");

            IAsyncResult result= printDelegate.BeginInvoke("Hello World.", null, null);
            Console.WriteLine("...");
            //BeginInvokeEndInvoke
            printDelegate.EndInvoke(result);

            Console.WriteLine("Press any key to continue...");
            Console.ReadKey(true);
        }

        public static void Print(string s)
        {
            Console.WriteLine(":"+s);
            Thread.Sleep(5000);
        }
    }
}

" more. "