Sharing session across rails apps on different subdomains
I think there is another SO question about getting a single cookie to work across subdomains that would answer yours:
https://stackoverflow.com/a/10403338/2573896
Also, I can imagine that using a memcached cluster with dalli and memcached as your session store would work as well:
http://awesomerails.wordpress.com/2011/08/23/rails-3-memcached-session-store/
For the purpose of your application, the first solution makes more sense though.