'rpool' is special with regards to the valid configurations we allow. With the rpool, you can only have single vdevs or mirrors. You cannot create a concat/stripe or raidz rpool. If your intention is to create a mirror, use:
You are welcome to create a new pool but one comment is that you can increase the size of the root pool
disk by outright replacing it with a larger disk or attaching a larger disk and detaching the smaller disk.
The steps are documented, here: