In the struggle to implement federated access in a relatively straightforward organisation, still not working, though we started back in the summer, I didn't stop to think about how it might work in more complicated situations.
The UK Federation website now carries some case studies, including one of my old institution, Brighton and Sussex Medical School. Based on interviews with Steve Holden and Sarah Howe of the University of Brighton, with the University of Sussex and the local NHS one of the three partner institutions who set up the school in 2003, it describes the difficulties of implementing federated access in an organisation whose staff and students are licensed to use electronic resources of all three partner institutions, as well as those licensed by the medical school itself.
http://www.ukfederation.org.uk/library/uploads/Documents/BSMS.pdf