Cloud images inherently come with one NIC. Once you create you cloud init image below create it in ProxMox as a machine, not a template and run it. This multi-nic image works for other providers as well. You will want to install netplugd to make this work.
sudo apt-get install netplug
sudo mv netplug /etc/netplug/netplug
sudo chmod +x /etc/netplug/netplug
Spin up the machine and add an extra network port in Proxmox and test it if you want. Otherwise you can turn it into a template.