I've always found the best way to do that is by downloading to one, then the other.
The more softwear you add to the process, the more chance of failures IMHO.

Simple with computers is best.
(then dropkicking )

I use Nikon Transfer to download images from CF cards, saving files onto my hard drive and designated backup, at the same time.

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Depending on your operating system and hardware setup you might be able to run RAID1 (drive mirroring) or use external storage like Drobo which is running raid already. Over the years I have had too many of the "cheap" RAID solutions fail so prefer to do things manually.

I have a couple of internal hard drives which are backed up to an external hard drive using a batch file and xcopy   This is scheduled to run every night via the Windows Task Scheduler. I then have a second external hard drive which is backed up the same way but kept off site.

xcopy c:\data\*.* /s /e /v /c /d /y d:\data\

You can substitute the c:\data and d:\data with whatever drives or subdirectories you like. This command will only copy the files which have changed since the last time you copied.

If you prefer to be able to run it when you want to, just create a shortcut on the desktop to the batch file. It is simple to setup, simple to check if working and free.