Depending on yr rqmts having 2 instances on the seperate server provides scalability.(allowing more connections and more transactions).
So one instance ats as a primary instance and the other as a failover instance.The failure/secondary instance takes over when primary fails.
This setup protects against instance failures etc but not against media/or non media failures.So don't think of it as an option for high availability against data failures* but scalability.
* here you might considering a stby database setup.
Hope that explains.
I'm a JOLE(JavaOracleLinuxEnthusiast)
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