You need an identity replication/management service like ILM/MIIS (
http://technet.microsoft.com/en-us/ilm/default.aspx). Essentially what it does is when a user logs onto one authentication system, it tells the other authentication systems who they are. The upshot is the user still gets single sign-on and all the hard work is done behind the scenes.
There are rival technologies that do the same thing and the user may already have one in place.
Leon Tribe
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