Outside, are you sure about the inline rear cockpits? I'm sure the pics I've seen of IAC TrIXs had the raised rear cockpit - I thought the inline mod first appeared with Nick Grace's wonderful restoration of G-LFIX in the 1980s and was copied for the rebuild of the sad G-TRIX (and others?).
I would have assumed that the rear office was the instructor's and was raised so he could keep an eye on things; As I recall, Nick Grace lowered the cockpit to try and keep the lines of the original aircraft and I suppose his idea was more to take people flying in a Spit rather than to instruct on it...