There are ways to do it with a network disk being present or something, but generally HA in Proxmox needs an odd number of nodes to reach quorum; basically if an HA node detects that it is isolated then it freezes all VMs, assuming it is having network issues and that other nodes, which themselves may not be isolated, could be running the same VM - since the whole point of HA is that if a node and its VMs disappear, the remaining ones take over duties until the missing node returns.
If you have an even number of nodes, you need a tiebreaker vote to reach quorum - half the total nodes plus 1 for a majority is the default.
You can adjust the total number of node “votes”that dictate what quorum is, but if you have two nodes and you set it to 1, then you’ll always have “split brain” where copies of the same VMs will keep running on both nodes, and if you set it to two then and node going down will freeze the other as well (both will assume they are the one with problems, since they’d both be below quorum). Therefore you need an odd number of votes.
The best way is to have a third host (or a 5th, or a 7th, etc. 😅); but there is a way (tutorial on Proxmox’s docs) to set up the presence of a network share as a tie-breaking vote, rather than a full additional node; the idea being that if the node can see the disk, that means it can see the network and therefore it is the node you’d want running the VMs.
So plan carefully around this, it’s not fun when a cluster you’ve become dependent on for services deadlocks itself 😅 ask my wife how I know this
I remember I got Jellyfin Lifetime for only $0.00 back in like 2018 before that started unlocking all their new features behind a nonpaywall