Transacting with different db tables


I am trying to make head and tail of a critical problem I am facing. I
have my railsapp which has its own user table and account management.

Along with that, I am installing Opinion forum as a subdomain on the
server. As you know, Opinion has its own user table and account
management. I want the user to login once and access domains and
subdomains without having to login again.

From an earlier post to his forum, I got the solution for having a domain wide cookie. So one part of the problem is solved.

There are some options I am looking at:

  1. When a new user signs up on the main railsapp, I need to insert a
    record (a kind of replication) in the opiniondb as well to keep the user
    account working. Is it possible to insert a record in another db?
  2. Short circuit the User model of opinion to authenticate users from
    railsapp’s db user_table instead of opiniondb’s user_table. (maybe Any
    opinion experts can help me there?)
  3. Any other ides for a Shared User model?

Awaiting expert suggestions and solutions