I went into this thread thinking that the setup didn't make sense, at least in terms of pressure differential (Heat mentioned above is just a driver in pressure differential, it does matter where the housings are placed to produce the correct differential to produce the desired airflow, but it was mentioned earlier) and it still might not... However, wrapping your head around it in a different manner, it might... If you look at the engine just running the 2 primaries as parallel twins, and don't take into account the GT55, you would have a very efficient set of turbos (more efficient than a traditional set of compounds), but due to the nature of a single stage, you are limited by your map. If you inserted the GT55 in the mix after the GT42's, and sized it properly to capture any excess energy (Using heat to create the differential), you could theoretically expand the map using almost no excess energy... So in otherwords, you aren't really compounding anything, you are just using the GT55 to widen the map of a single stage turbo setup by putting the turbos in their sweet spot for a longer amount of time, simply by playing with differentials....
(Scratches head...)
Kinda makes sense I guess??