In a 2 for 1 rotation you have three people in rotation covering those two jobs. The “lead captain” is master for his whole hitch, the “relief captain” is chief mate half his hitch and master the other half. The “third captain” is chief mate his whole hitch.
That was never the question. He said he was an OICNW but since he was the only person onboard besides the master with a master’s license then he should be considered chief mate.