If you mean bi-directional in the form of transactional replication, you should go down the route of peer to peer replication. I'd stay away from updatable subscriptions as that's not true replication.

http://www.sqlskills.com/blogs/paul/sql-server-2008-configuring-peer-to-peer-replication/[/url]

http://technet.microsoft.com/en-us/library/ms146914(v=sql.105).aspx

The alternative to this is merge replication.

http://technet.microsoft.com/en-us/library/aa337473.aspx