You can be logged in only in one of the domains at the same time since the authentication details are persisted in a cookie and cookies are per domain. This is currently the expected behavior. Please let us know if you think that the experience of any particular scenario can be improved is some way.
We have not tested with Thinktecture. Our single sign on is also based on Windows Identity Foundation and it is expected that they will be compatible. Note that Sitefinity is only taking the authentication from the STS and it will not care if any profile or authorization information is in the token.
the Telerik team
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system and vote to affect the priority of the items