You may have trouble keeping a quorum with a proxmox cluster of 2. You should really have 3, or set up a q-device. (Source)
Each node in a cluster has a “vote” in a healthy cluster. If one of your devices goes (or is taken) down, you’ll lose quorum, and the GUI and some other stuff. You need 3 votes for things to work reliably. I have an old RPi set up as a q-device and it works fine.