You have suggested me to setup one more Proxmox as cluster members right?
Not necessarily, you might run single node all just fine, in that case however you really want to keep at least the contents of /etc/pve backed up (see the links above, some discussed other locations). What I meant to say was that if you worry about crashing/failing node, which already happens to be in a cluster, then the shared configuration data is actually replicated across nodes. I also want to point out that running just 2 nodes is not a good idea, it's a whole separate topic you will need to look up regarding cluster setups, but either you want 3+ nodes or if you must to have 2, preferably a Q device. See more for details:
https://pve.proxmox.com/wiki/Cluster_Manager
Sorry, maybe I should take a look at the link that you provided me
For the backups, it really was discussed all over there.
1. For cluster environment, I just need to setup Proxmox and rejoin the cluster. After that, all data should be replicated from the other member.
Is this correct?
The shared configuration data, yes. You still have to have backups of your VMs, etc. You might be replicating them (requires ZFS with Proxmox VE), but then still it is not a backup. In fact the configuration is also good to be backing up - because consider also user error, e.g. you delete config which you did not intend to, it is instantly replicated, i.e. gone from all nodes in that case.
2. The cluste members (machines) should have the same spec? For instance, the same CPU model, the same amount of RAM size and vendor, the same model of motherboard vendor, etc.
Not necessarily, definitely not strictly, but it helps if they are similar, especially in terms of networking.
No worries, I just want to emphasise that cluster is not a backup a 2 nodes are nota good cluster, more you will have to start reading up. For backups alone, the original links were actually good to look at.
