Steve is right, the mirrors MUST be on a separate disk spindle.
Oracle redo log mirroring allows a redo log can be simultaneously copied to multiple disks at the same time by the LGWR process.
This ensures that the loss of a group of log files will not affect operation. Groups are archived together. The MAXLOGMEMBERS parameter in the CREATE DATABASE statement determines the maximum number of redo logs in a group. The MAXLOGFILES parameter in the CREATE DATABASE statement determines the maximum number of groups. The reason that the MAXLOGFILES parameter is MAXLOGFILES and not MAXLOGGROUPS is that it is a carryover from the days before you could have multiple log members, as well as multiple log groups.
See here for more details on redo log miroring:http://www.dba-oracle.com/t_ault_27_lgwr_m...maxlogfiles.htm