Mac mirroring and clean install

[kaia]

So I think I accidentally mirrored my 2 internal hard drives, or something. When I add or delete a file to one, it does it to the other. I must have done this when I set them up. In any case, I now want to do a clean install (update the operating system), and I'd like to separate my hard drives so one of them can be a backup drive. Does anyone know the process by which I would

1. Make one of these drives the backup drive
2. Install the OS onto the other one

Or do I install the OS onto both, or what? I just want to make sure I have a backup of my data.

Thank you
Kaia