Web Forms :: 2 Systems Not Works Simultaneously

I have 2 Web applications in the same IIS with the virtual directory structure below:

http://www.mydomain.com/site1
http://www.mydomain.com/site2

both site1 an site2 are using form authentication and creating the token further adding in cookie as below,

FormsAuthentication.Initialize();
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1,username,DateTime.Now, DateTime.Now.AddMinutes(20),rememberMeSet,globalID ,FormsAuthentication.FormsCookiePath);

// Encrypt the ticket.
String hash = FormsAuthentication.Encrypt(ticket);
//Create the authentication cookie
HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, hash);

My problem is when I'm logging in to Both site1 and site2 then these two are not running simultaneously on single browser using different tab for same user name.one is being logged out when logging to the other application and vice verse.