Sorry for reposting to my last post but just got the status where I could see the pictures, been a few years since I posted here.
The transition you show is problematic. You will get material build up in the large duct and that is at minimum a fire hazard and possibly a weight hazard depending on how the duct is supported. Most ductwork is designed for 50% full, but in your situation, you could easily exceed that going from a 6" to a 16" duct. The transition being eccentric with flat on top also promotes material dropping out of the airstream.
Did they do this to cut the system flowrate? Are you only using one machine at a time now? I'm curious as to what they were trying to accomplish.
Mike