(How) Can I configure a fallback for DDClient?

I [want to use the Fritzbox update feature of DDClient](https://github.com/linuxserver/docker-ddclient#get-dynamic-ip-from-fritzbox) , but also want to have a fallback to a public web server, if that does not work for some reason (e.g. cause someone just switched to another router or whatever…). As far as I known, just duplicating the use=cmd key seems to override the previous configuration?

protocol=dyndns2
use=cmd, cmd='curl https://checkipv4.dedyn.io/ '
use=cmd, cmd=/etc/ddclient/get-ip-from-fritzbox
#use=if, if=eth0
ssl=yes

server=update.dedyn.io
login=domain.dedyn.io
password='___password_here____'
domain.dedyn.io

The syntax use=cmd, cmd='curl https://checkipv4.dedyn.io/ ', cmd=/etc/ddclient/get-ip-from-fritzbox also works, but I guess it does the same, or does not it? In any case [the official documentation](https://ddclient.net/usage.html) is of little help and does not explain the configuration format in a very elaborate way. So can you actually specify a fallback in ddclient? If so, how?