That depends on what level of HA you want to end up with.
If you want proper HA, you'll want to plan on adding a (small, like a Raspberry Pi) third node for quorum. If you are already taking backups and you just want "I can restore on the second system" then it's slightly simpler, but mostly the same process:
- Setup new node, add to cluster
- Migrate all VMs and LXCs to new node
- Remove and upgrade other node
- Add rebuilt node to cluster
If you're planning on proper HA, I'd strongly advise having the proxmox installation on a second small drive on each node and leaving your 1tb drives as data only.
This article half-explains one option for a two node setup (zfs replication), which is functional but not ideal. If you want to get your feet wet with Ceph then I can give you some pointers.
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