A little confused... the 2-6-6-2s are compound engines, the rear cylinders feed the exhausted steam to the front cylinders then out the stack. How would this make any alteration to the sound (if it were even possible for the sound file to account for the out of sync drivers)? Should be 4 chuffs/ rev like a regular steam engine.
Simple articulateds (4 independent cylinders, Challenger, BB, etc...) would drift in and out, but I dont know if the sound file has the ability to adjust. GRJ probably knows if there's a switch or hall sensor on both sets of drivers...