Air Conditioning - Need help with AC differential

I need some advice. I have an 18 year old 2 story home (about 3500 sq. ft) located in PA which is heated and cooled by a 5 ton Trane heat pump with air handler in the basement. While running AC, I generally see a 4-6 degree temperature differential between the first and second floors. I have tried the usual adjustments such as closing first floor vents, however it is not a very effective fix. I realize if I was building a new home, 2 floors=2 units. However we really like this particular house and prefer to stay, so what is the best solution? How involved and expensive would it be to add an air handler to my attic (which is trussed) and drop AC in the second floor rooms through the ceilings (4 bedrooms)? Does it require a second compressor? I have been told there are booster fans that can be added to the ducts which service the second floor rooms? Would this be an effective and less costly fix? Are there any other fixes? Thanks in advance for any suggestions.

You can try running the units blower 24/7. That will sometimes help even out the temps. Another thing to look at is if you don't have any cold air returns on the 2nd floor it would be a good idea to add them.

The idea of a 2nd unit for the upstairs is great and it normally isn't a tough job with attic space. Yes it would require a 2nd condensor outside. The downside is it will cost you. When you add the 2nd unit for the upstairs this will in turn cause the 1st floor 5 ton unit to be oversized and it will need replaced also with a smaller unit.

Forget the booster fans. 90% of the time they are useless.

Thanks Mattison.

My dad's house has 2 floors, and he has two units in the house. The upstairs still remains a bit warmer because heat will rise up to it, effectively taking it away from the first floor. Average out the two floor temps, and that's the temp your house would probably be with only one floor.

Two units may not help the temperature difference, but it will help cool it down