FWIW, I've always gotten the impression that with the possible exception of the Bristol, the usefulness of two seaters in a dogfight depended entirely on the skill of the pilot and the marksmanship of the observer.
Two seaters could be absolutely formidable when well flown - or easy meat if not properly handled. I knew a DH9 pilot who sideslipped his bomber out of formation onto the tail of a Fokker DVII and filled it full of lead, an RE8 pilot who took on 3 DVII's and sent one down while making good an escape from the other two, and
Doug Campbell said he had no idea that his first victory was a fighter... he thought he was fighting a two seater the whole time! Just goes to show ya that sometimes you could hardly tell the difference... especially rookies.
And several two seater pilots said that there WAS a difference in observers. Having one that was a known sharpshooter was considered a big blessing that may well affect your life span.