Databases bidirectional replication in same SQL Server

This is the scenario:

We have 2 applications, each one connected to it's own database. Both databases are in the same SQL Server 2008 R2 Standard Edition. Both databases have several identical tables (TableX, TableZ in the following example) which we need synchronize:

1. AppA writes/reads in DB1 (TableX, TableZ)
2. AppB writes/reads in DB2 (TableX, TableZ)
3. Bidirectional Sync TableX and TableZ every 8 hours. During the synchronization both databases must be availables.

I'm reading "Selecting the Appropriate Type of Replication" but I don't have clear which one and how to use it in this situation (merge replication??)