This is a double deck with a air slot at the top. The air slot on top is to keep the air speed in the upper part of the diffuser from dropping. When I tested a smiliar setup in solidworks I could see that the air tends to separate and slow down at the top of the upper diffuser. The air in it tends to flow straight out (horizontally) instead of angling up and out so that is the "belly" part is cut out. It's just to keep the air speed up.
Notice the slot? ;look at the bottom edge of the Red lights, you can see right through it.
It is still two diffusers though. Remember a diffuser is a gradual tapered enlargement in cross sectional area from the entry plane to an exit plane. The uppermost piece can not be a diffuser because it has no lower border: if you look at the lower edge slot it is an abrupt opening into the flow of the second diffuser.
I don't think this diffuser is related to the other diffusers shown because of this slot.
I could be wrong but I have never seen a diffuser like this used during the season.