Sharing one session data for multiple site

Rails
1

eg:

  1. site A (www.a.com) and site B (www.b.com) share the same database
  2. user X log in into site A, stored in a session data
  3. user X enter site B, old session data from site A for user X loaded
    for site B, and user X is now logged in

can it be possible? how?

thanks in advance

best regards,

adrian liem