Copy to same drive or separate drive? Is there a benefit to copying (with Carbon Copy Cloner) large amounts of data (400 GB) from one physical drive to another (both WD My Passport external USB 3.0 HDD), rather than to a separate partition / volume on the source drive?

I imagine that using the same drive might be slower, since it must do both read and write. While with separate drives, each drive does only read or only write, so maybe it's faster.
Copying data to a different drive negates the risk of losing all your data due to physical failure of your primary drive (as would be the case if you merely partitioned it and used the other partition as a backup). 

Since that’s the principle cause of data loss (way easier to repair a corrupted partition scheme than a broken HDD/SSD), it makes sense that copying data to a different physical device is the most effective method of backing up — it’s the only one in which true redundancy exists.