Single Sign on using ASP.NET Form Authentication Cross Domain

I have a client that recently converted a website to use one source code base for multiple domains that point back to the same website. The web application acts as a portal for properties to be managed.  The user may have one property or many they can manage.  The user can go to any of the properties urls at first and login.  If the user has many properties to manage the dropdown box will be loaded with the other properties. When the different property is selected from the dropdown box is the url information is grabbed from the database then a Response.Redirect is done in the code behind page.  For example Current URL logged into (www.hilton.com), the user selected (www.springmanner.com) from the dropdown box.  What happens is that even though, the selected URL still points back to the same website. The Login control using Form Authentication asks the user to login again. The FormAuthentication is loss because the switch in Domains, eventhough the different Domains still point back to the same website IP address.  I have read how to achieve this if the domains where sub domains that seems pretty straight (i.e. rabbit.house.com, bear.house.com).  The application uses InProc for session mode, I realize that this mode may not work for this scenario. How can I accomplish this using ASP.NET without using Windows Live ID or OpenID? Thanks for your help in Advance!!