The Break Duct command will break a single duct centerline into two ducts.
To break a duct, go to
Ribbon: DM HVAC->Ducts-> 
Break Duct
Pulldown Menu: DM HVAC->Duct Centerlines->Break Duct
You will be prompted to specify the location of the break on the duct centerline.
Select duct at break point:
The centerline will be broken into two segments. Both segments will point in the same direction as the original centerline.