The Force Ducts Together command is used to connect two separate duct centerlines on the drawing and in the database. This will adjust the angle for one of the ducts. Use the Fillet Ducts command to connect ducts without adjusting their angles.
To connect two ducts that are on the drawing but not currently connected, go to
Ribbon: DM HVAC->Ducts-> 
->Force Ducts Together
Pulldown Menu: DM HVAC->Duct Centerlines->Force Ducts Together
You will be prompted to specify the duct that is closer to the fan.
Select duct or label that is toward the air handler or fan:
Then you will be prompted to specify the duct that is closer to the diffusers.
Select duct or label that is away from the air handler or fan:
The end points of the ducts will be moved so that they are connected.