Changing the Log Mode from SINGLE to DUAL 
Use
You can change the
log mode for the current
database instance.
Prerequisites
There are no open parameter sessions (
Closing a Parameter Session).The database instance can be in any
operational state.
The current log mode is SINGLE.
Procedure
Under New LogMode select option DUAL to mirror the
log volume.
If no mirror volume has yet been defined for the log volume, you must do this now:
To integrate the mirror volume in the database instance, you must restart the database instance. If you are not in the OFFLINE operational state, select the option
Restart Database and reintegrate mirror volumes implicitly after LogMode change.
Confirm your entries.
Choose LogMode ® Change LogMode.
Result
The new log mode DUAL is displayed as the current log mode.