I am backfeeding an existing 200 amp panel from a new 400 amp 1ph 120/240v service that is 60 feet away. The existing panel has a 200 main breaker. I am planning on a 200 breaker in the new 400 amp panelboard to protect the subfeed conductors (SER 4/0 AL). Are there any NEC rules against having more than one overcurrent device in the subfeed circuit?

No there isn't.

I am curious though. Are you the owner? Contractor? Electrician?
Anyone doing work on a scale like this should NOT have to ask these questions.