You can have multiple repos connected at the same time but make sure that they are using different config files - --config-file flag - otherwise repository.config is always used.
AFAIK you can not start second serve on the same host but not sure though why you would want it.
To have your repo A backup the easiest would be to periodically sync it to another drive using e.g. rsync.
On the other hand IMO if this is another disk on the same server I think you would better use it as a RAID 1 with BTRFS or ZFS.
I was trying to do backup of a repo at one point until it was suggested here that I should just use the kopia sync-to command to duplicate the repo elsewhere. That worked well and seems more efficient.
Could you consider the same and create a separate repo for the server files? (abc, xyz) Thus end up with two additional repos i.e. Repo B (duplicate of A) and repo C (abc, xyz host files).