I did setup the mirror before formatting venti/fossil partitions.

Otherwise, you'd need to copy one disk to another while the partitions
are idle.

Let's say you have /dev/fs/mirror out of /dev/sdC0/part and /dev/sdC1/part

If you add /dev/sdD0/part you could dd from either /dev/fs/mirror or
from /dev/sdC[01]/part to /dev/sdD0/part. You should also add the device
to devfs.

I can help somehow, let me know.