Append Tee & Boot to Existing Branch ( flexible duct) to Split supply between floors
Splitting that 7&quot; flex duct to two ducts will yield very little heat to either room. I would find another &quot;tap-in&quot; spot off the main trunk to get heat for the basement. Can you add a supply register to the bottom of the supply trunk.
A picture or two would help us understand what you want to accomplish, what problems you are encountering, and what options we see that would resolve this problem.