Hi.
Have you considered using Oracle Golden Gate or Oracle Data Guard to replicate between two Oracle DB instances? I think that's the easiest way...
You can also use Repilcation Server (Heterogeneous Ed. with Oracle Express Connect) to achieve zero downtime. Please read these articles from RepServer 15.7.1 documentation:
1. Oracle as Primary Data Server
2. Oracle as Replicate Data Server
3. Heterogeneous Warm Standby for Oracle
4. Oracle Replicate Databases Resynchronization
Regards.
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Marcin