It's pretty simple. :)
On Login Button Click, Store the type of user as well in the Session. As you've said that Login pages are different for both the users, then it won't be an issue. For example,
If Doctor is logging in, on button click of login,
Session["UserType"] = "doc";
And on the other page, do something like following,
protected override void OnPreRender(EventArgs e)
{
base.OnPreRender(e);
if(string.IsNullOrEmpty(Convert.ToString(Session["USERID"])))
Response.Redirect(ConfigurationManager.AppSettings["InternalUrl"].ToString());
if(string.IsNullOrEmpty(Convert.ToString(Session["UserType"])))
Reponse.Redirect("SomePage.aspx");
}
-KR