RAIDZ has 3 or more disks; if you have two disks, you have a mirror. Your two disks comprise one mirrored vdev, and your pool consists of that one vdev. To increase the capacity of your pool, you can (1) replace your disks with larger disks, one at a time, following the manual's instructions for replacing failed disks--once you've replace both disks, your vdev capacity (and thus your pool capacity) will increase to reflect the larger disks; (2) add more disks, which will comprise one or more vdevs and will be striped with your existing vdev to expand the capacity of your pool; or (3) some combination of (1) and (2).
If you have two more 2 TB disks, and want to add them to your pool and maintain redundancy, here's how you'd do it:
- Install the two new disks in your server
- Log into the web GUI
- Click the Storage button
- Click the ZFS Volume Manager button
- In the "Volume to extend" dropdown, choose your pool
- Click to add the disks
- Make sure the dropdown is set to "mirror" (which it should default to)
- Click the "Extend Volume" button
The system will create a mirror vdev using the two new disks, and add that vdev to your existing pool. The capacity of your pool will be increased by the capacity of the new vdev, about 1.8 TB. This process is irreversible--once you've added that vdev, you can't remove it. You will still have redundancy in your pool--any single disk can fail, and you won't lose any data. Two disks can fail, but only if they're the "right" two disks--they would need to be one from the old mirror, and one from the new mirror. If both disks in either mirror failed, you would lose all your data.