if a sparky has to work in the sub and wants to shut it down you have to throw the main?
for a $20 breaker is it not better to have a 2 pole breaker on the feeder to the sub?
But this is an NEC forum and I am going to give an NEC answer to the best of my ability.
i still dont think this satisfies 240.21 as each ungrounded conductor requires an ocpd at the point it gets its supply.
The overcurrent protection for the feeder is at it's point of supply, no part of the 2 AWG conductor is ahead of the circuit breaker.
the main is a ocpd for the entire panel and acting as the service main not a feeder ocpd
There is no NEC restriction preventing us from using that breaker for both.
Current is current, the service main is as capable of protecting the feeder as the panel.