Add the same key to a retired PIV slot of two different Yubikey's

I am using the yubikey plug-in for age (https://github.com/str4d/age-plugin-yubikey). It creates a private key and writes it to one of the retired PIV slot. What I want to do is to write the same private key to two different Yubikeys.

One possibility is that I generate a compatible key pair using a different tool and use yubico-piv-tool to import to both keys. But that is not very convenient.

Does anyone know how to install the same private key to two different Yubikeys using age-plugin-yubikey?