We're using DPM 2010 for exchange backups,
The backup Disk(s) are iSCSI attached drives from multiple NAS boxes.
We'd like to mirror iqn.2009-07.com.example.example:RAID.iscsi4.vg0.iscsi05
onto iqn.2012-3.com.example.example:RAID.iscsi4.vg0.iscsi05
DPM 2010 requires the disk for itself and handles volume creation, Therefore we can't just create a mirrored volume in Disk Management. DPM itself doesn't seem to have any ability to mirror the Disks in its storage pool.
The disks are initiated using the Windows-server-2008 iSCSI Initiator, the two iSCSI drives are on seperate NAS boxes (one of which will soon be down for maintenance)
Any tips on how to mirror the volumes from one drive to the other?
Inspect the RAID capabilities of the server and its storage controller. Add a disk to the controller and mirror the disk that DPM uses over to the second disk. I think it would be preferable to mirror at the disk level rather than mirror volumes.
You should consider what you are trying to accomplish by mirroring the backup volumes.
If it is for disaster recovery, mirroring the volumes by themselves is not a complete solution. DPM creates hundreds of dynamic volumes on a disk and manages them exclusively, as you know. But in order to do a restore, you must also restore the DPM DB, and of course the DPM server itself.
DPM 2010's DR protection strategy reflects this - you spin up a second DPM server and replicate the data between the two. This protects your data and the DPM settings/configuration necessary to restore it. It makes DR much simpler.
If you only want to mirror the volumes at the storage level, see if your NAS boxes support replication.
One other note: the "volumes" you quoted above (beginning with iqn) are not actually volumes. They are iSCSI targets, each of which is identified by its own IQN. Volumes are presented over iSCSI targets, but the relationship does not have to be 1:1. You can have multiple volumes on the same target.