From my understanding, this is how most multi unit A/C ducts are layed out. Basically a racetrack with square cormers. Not a real efficient design and why racetracks aren't designed with 90 deg. corners and pit roads.
I'm assuming this is how mine is, as I have two parallel ducts running front to back that extend past the first and last vent on each duct and some air comes out of all vents with eiter A/C rinning independently. I plan on getting out the bore scope this winter to verify. (Image 1)
These images were pulled from th GD site and I plan on trying to replicate it on my 2 A/C unit system this winter. (Image 2)
While your checking things out, look for any air leak points in your A/C plenum and at each vent. Mine was blowing a lot of cold air into the ceiling atic cavity. Fiol tape is your friend.