Sorry, I realise this is an old post but I have the same question but I'm not sure the answer above works.
I also have two separate servers running Proxmox, both of which are running fine. I (like the original question) would like to be able to manage both servers from the same view i.e. have two servers listed under 'Datacantre'. I went about trying to create a cluster on one of the machines which was fine but when adding the second, it correctly reports that the second node already has VM's so can't be added.
I've now seen the notes about how to add the second node but that involves backup/restore etc. I'm not that interested in clustering (I had enough of that with VMS) but would like to be able to manage both servers from a single window. Is this possible without creating a cluster?