SharePoint 2010 SSO with ADFS and Custom STS
===

I have the following Scenario:

Three different Sharepoint Server: sp1.company.com; sp2.company.com...
Internal Users using Windows Authentication and external ssers using Forms Based Authentication (FBA).

When the external users change from one server to the other they have to login againg. My goal is to have a Single Sign On (SSO) for all the SharePoint Server.

I thought maybe I can do this by using ADFS for the internal users and a customSts (ThinkTecture IdentityServer) for the external users. 

Would this be possible? How do I setup the Sts servers ADFS and IdentityServer? Do I have to connect ADFS to IdentityServer over WS-Federation?
