Noob Question: How to Mirror a Root Pool

[linux40]

I migrated from my old ext4 partition nvme0n1p2 to a new zfs pool as the root. The status is as follows:

$ zpool status
  pool: zroot
 state: ONLINE
config:

        NAME         STATE     READ WRITE CKSUM
        zroot        ONLINE       0     0     0
          nvme1n1p2  ONLINE       0     0     0

errors: No known data errors

Now I want to mirror nvme1n1p2 with nvme0n1p2, but got the following error via zpool attach

# zpool attach zroot nvme0n1p2 nvme1n1p2
invalid vdev specification
use '-f' to override the following errors:
/dev/nvme1n1p2 is part of active pool 'zroot'

Adding -f does not work here. So do I have to boot another OS with the zfs support and operate the pool offline? The operations in the Oracle document seem manipulating online, and by searching "is part of active pool", I see people faced more complicated errors than I have.

[linux40]

My fault. I typed devices in the wrong order.