The Fillet Ducts command is used to join two ducts together. The command works similar to the standard CAD FILLET command.
To fillet two ducts, go to
Ribbon: DM HVAC->Ducts-> 
Fillet Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Fillet Ducts
You will be prompted to select the first duct to fillet.
Select duct or label to fillet:
The selected centerline will be highlighted. You will then be prompted to select the second duct.
Select duct or label to fillet:
The two ducts will be extended to each other and connected. If the ducts are at different elevations, a vertical piece will be inserted at the elbow to complete the connection.