1-You need install a SQL Server on one of your tills. This will be master that keeps database.
2-You need to configure SQL Server so that your second till can see the master.(Check forum for SQL Server configuration)
3-Your connection string should be like that for both tills "data source=ComputerName\SQLEXPRESS; user id=username; password=password"
4-You need to enable Samba Messaging server if you want both of your tills updated at the same time.