That's been our problem from day one. I assume the duct work was designed well at the time, but now trying to have 2 bedrooms up here with crappy ventilation...
We would need to have some massive work done, or a 2nd unit somehow like you pointed out. But with the total cost of this plus the compressor + labor... and then we just replaced some industrial sump pumps outside & had electrical work done and the motherboard replaced for the unit to run the pumps outside.
The good news is that we officially have heat back on today!! Been back on since like 10am and feels warm already but thermostat is still at 62 for now. Was down to 53 the other day! And no more blowing circuits from space heaters around the house.