So I've been trying to get this solved, and my solution has now changed slightly.
I currently have my iphone backup the camera roll to external drive "N:", I would also like the backup to go to external drive "M:".
For obvious reasons both drives will never be attached to the pc at the same time (in case of a power surge I don't want to lose both drives).
How can I achieve this? I have one pc, 2 drives, and 1 iPhone camera roll.