This pasts summer, I had severe condensation on a section of ducting between my first and second floors. This condensation was worst in an area where I have a utility chase running from the attic to the basement. I assume I have a return duct in this chase. At the advice of some HVAC contractors, I had a few feet of ductwork insulated where the problem was most severe (only what was behind the ceiling drywall that had to come down anyway due to water damage). I also had my attic sealed and re-insulated since a few contractors thought that air from the attic was getting into the chase and contacting with the ductwork between the first and second floors. I assume the test for this fix will be in the summer when we're using the AC again.

However, today while cleaning, I removed the vent cover from the return duct (on the first floor, below where the condensation was occurring. Other than finding a ton of dust and dirt, I noticed a large gap in the ductwork where some metal seemed to be dividing the duct in two (pics are here). This gap is about a 1/2" wide at the biggest and runs the width of the duct.

Does this look normal? Is there any chance that this could be contributing to the condensation? I also have poor air flow to my second floor, could this be contributing to that as well? I do know my system's delta T is on the high end (around 22 degrees). My HVAC contractor played with the fan speed, but only was able to lower this by a degree or two from around 25 degrees. I wasn't sure if this gap may be contributing to this issue too.