Given this information that Homebild caught later; but when said first that you had the cap change, would have nailed down the answer that your two contractors didn’t know enough about:
“There should not be a single vent cap or ‘shroud’ covering these two appliance vents.
Each must be capped and terminated separately.
To install a single cap over both vents violates the listing of both appliances and should be removed.”
http://www.HeatProUS.com energy businessmen’s knowledge
Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.