2 Rails Applications sharing same login system

You can't share cookies between domains, but you can use page
redirection coupled with an authentication table in the backend to
sync cookies between domains. Here's brief writeup on how 43 Things/
Places/People share auth:

http://dev.robotcoop.com/Technology/xs_auth.html

In that case we did have the "luxury" of sharing the same database, so
the apps could access the same auth table trivially.