Open the config from the container you want to edit, you'll find the exact path by clicking on the container icon -> Show Config and in the following popup the first line shows the path.
Edit the path with whatever you want and create a second entry for your second NIC:
# Network configuration for second NIC
lxc.net.1.type = veth
lxc.net.1.flags = up
lxc.net.1.link = eth1
lxc.net.1.name = eth1
lxc.net.1.hwaddr=AVALIDMACADDRESS
This example is for veth only!
lxc.net.1.link is the physical link on your Unraid server, lxc.net.1.name is the interface name in your LXC container.
Please also make sure that you add a valid MAC address to the second NIC.
Hope that helps.
BTW, I would strongly recommend to upgrade, 6.11.5 is outdated.