Although a two node cluster is not recomended for HA due to split brain problem (see Two-Node_High_Availability_Cluster in Proxmox wiki for more info with this config), you can set it up in Proxmox for a basic cluster usage. A two node cluster has an special cman configuration in order to maintain the quorum when one node is not available.
To configure a two node cluster in proxmox, copy /etc/pve/cluster.conf to /etc/pve/cluster.conf.new and edit the new file changing the following line:
<cman keyfile="/var/lib/pve-cluster/corosync.authkey" two_node="1" expected_votes="1"/>
Now, you can activate it through the proxmox web interface in datacenter -> HA, review the changes and activate them for the two nodes. Now you can restart one node without losing cluster quorum.