Sight unseen I would address the house problem this way.
Add a unit upstairs and duct it into the existing duct (disconnect the upstairs ductwork from the downstairs)
If the garage isn't used for cars, I would add a drop and return in the garage from the downstairs unit to take care of that.
All this depends on the garage not housing cars. Here in soflo (and i'm sure most places) its illegal for us to recommend and install duc work that connects the house to the garage. Carbon monoxide poisoning being the main issue.
There is really so many options and its fun to tinker and hear other peoples ideas. Good luck!
Add a unit upstairs and duct it into the existing duct (disconnect the upstairs ductwork from the downstairs)
If the garage isn't used for cars, I would add a drop and return in the garage from the downstairs unit to take care of that.
All this depends on the garage not housing cars. Here in soflo (and i'm sure most places) its illegal for us to recommend and install duc work that connects the house to the garage. Carbon monoxide poisoning being the main issue.
There is really so many options and its fun to tinker and hear other peoples ideas. Good luck!