Can I produce two copies of a backup?

Windows 7 x64, Chrome

For a couple of years now I've been using Windows Backup to backup to an external disc drive, then swapped EDDs and produced another backup.

I wasn't too keen on Windows Backup because I couldn't see what files had been backup up.

I tried SyncToy, which worked well :

Run one, X files copied or created, swap EDDs :
Run two, zero files copied or created.

A google search revealed that incremental backups unset the archive bit, while differential backups don't (that was my understanding, anyway). So the first run backed up changes, etc,, and unset the archive bit. Obviously the second run found nothing had changed.

Does this mean that it's impossible to run two copies of a backup? My solution on the SyncToy backups, all of which were in subfolders of one higher level folder, was to copy the entire backup to a the second EDD, using a single copy/paste.

Having deleted the old folders on the second EDD first, of course.

I've now registered for the free 5GB storage with iDrive, which is working well. However, I'd still like a local copy.

So back to the original question - can I run two copies of a backup?