Replication
Peer-to-peer replication: http://msdn.microsoft.com/en-us/library/ms151196(v=sql.120).aspx
Merge replication: http://msdn.microsoft.com/en-us/library/ms152746.aspx
Replication Identity columns: http://www.replicationanswers.com/p2pidentities.asp
How to setup peer-to-peer replication:
https://www.youtube.com/watch?v=Ld_bwHe8kcQ
http://technet.microsoft.com/en-us/library/ms152536%28v=sql.105%29.aspx
Handle error/conflict:
http://www.replicationanswers.com/P2PHandlingConflictsAndErrors.asp
useful articles:
http://www.codeproject.com/Articles/715550/SQL-Server-Replication-Step-by-Step