Overlap one side as you normally would and create a butt joint on the other.
How to install ductwork design in attic.
Use metal flex or fiber board ducts that are insulated to code.
Make the layout as symmetrical as possible with branch ducts spaced evenly along the trunk duct and each branch of similar length.
Proper planning for the path and placement of the duct trunks and branches in attics during the mechanical design phase as well as during installation is a recommended practice to avoid crimping.
Situate the air handler close to the center of the layout to minimize the length of ductwork spans.
The vent should be installed under the first course of shingles and sealed to the sheathing and installed according to the manufacturers specifications.
When installing the roof vent try to keep it the most direct route to the outside preventing sharp turns in the ductwork where condensation can build up.
When hvac ducts are installed in a vented attic in a dry climate bury the ducts in attic insulation to protect them from temperature extremes in the unconditioned attic space.
Additional considerations need to be given to all components in particular ducts that are installed in floor joist bays and wall chases.
Install ductwork so that it is in direct contact with i e laying on the ceiling and or truss lower cords.
The duct insulation should include a vapor barrier cover.