Duplicated saves with borgbackup

I need to backup data using Borg Backup from a remote server to a local server (raspberry-pi like - it is actually Yunohost) equipped with a 2 slots storage bay. I would like to have identical copies of the backup data on each disk. Any idea how to do so? I've never done anything like this. Should I create two repositories, one on each disk, and check on Borg's settings to sync them? Or should I set a software raid 1 between both hdds?