This is from Metalink: I had similar situations but needed the
additional arch process:

CAUSE
------

'Failed to archive log' messages appear in the alert log as multiple arch procresses are started
They are trying to start archiving of filled redo logs but as another arc process
already had started archiving them, other process faile and simply write a message in the alert log.

If you look atthe alert log , you will observe that, the arc process that starts archiving the log
is the process which eventually completes it


FIX
-------

This messages do not cause any problem to the working of database
Except that they fill up the alert log quite fast!!

to stop these messages from appearing in the alert log

decrease the archiver processes equal to 1


..... Please note, its still a draft.

Regards,
Rowena Serna
Oracle Corporation




HTH
Gregg