This post reminded me to look for open source code to handle multi-logins, I'm currently in the planning stage of a project. These seem quite comprehensive. Is there any reason why I would be better off rolling my own code?
http://www.omniauth.org (Ruby/Rack)
https://github.com/bnoguchi/everyauth (node.js)
https://github.com/ciaranj/connect-auth (node.js)