Database archive log placement on SAN storage

Assuming duplex archiving means writing the same transaction log to 2 seperate physical files, then the answer (in my experience) is that this is not a normal occurence especially in a SAN environment. You do your mirroring at the array level to match your risk threshold. (i.e. you can do 3 mirrors, remote site mirroring, etc.) Avoiding doing duplex archive logs cuts-down on some overhead to your host by letting your storage array (which is optimized to do it) handle the extra write overhead.

As for 2) it is definitely in your best interest both for logistical (backup, recovery, replication, etc.) and performance reasons to have your archive logs and core database volumes isolated on different volume groups and physical drives. This is definitely a best practice and should be followed for all but the smallest of database environments.