I’ll just assume that kanicbird has a valid point that’s not being explained very well, because moving heat around works by one fairly simple set of rules, and “hot water does not freeze faster.”
As for adjusting vents and flow, it can do amazing things, but it needs to be adjusted systematically, with an understanding of what the whole “system” is trying to do, and each adjustment needs to be given time to balance. To restate what I said in my first post, AC does not “blow cold” the way a heater “blows hot.” Sucking the heat out of a residence is a complicated, balanced, finicky process. Assuming the AC is both in decent condition and was sized correctly, it should be possible to balance the cooling flow so that both upper and lower floors are comfortable. Maybe not perfect - but comfortable. It takes time, patience and understanding to get there, and part of that understanding is that AC pretty much has to run 24/7 and can’t be switched on and off like a spot heater.
The only house I’ve ever had that I couldn’t balance this way had an office (second master suite) with high sun exposure and three high-powered computers that ran continuously. I did have to run a window unit in that room some summers.
It’s possible the OP’s AC is substandard or otherwise flawed. But I suggest that a little more time trying to make the central AC do its job will pay bigger dividends than trying to run a separate unit that will likely ‘fight’ with its big brother and make the power company happy.
It cost the same to remove 1 BTU of heat at 72 degrees as it does at 100 degrees. Exhausting the air from a AC unit inside a building means that you have to pay more than double to remove the heat out of the building.
1st energy is used to remove the heat from the 1st room and that heat plus extra energy a AC unit uses, No AC unit is 100% efficient.
2nd the heat from the room plus the heat from running the AC unit is removed from the town home.
If you system is large enough it is not necessary to run 24/7. Most office building do not run 24/7. Most buildings that I worked in AC hours were 6:00 AM to 6:00 PM Monday through Friday. Depending on the out side air temps the building start times were from 4:30 to 5:30 AM. and the buildings were down to temp by 6:00 AM. If not we got customer complaints.
I own a 2 story house and with out making changes the 2nd story will be 10 degrees above the 1str floor. It is a matter of bad air flow. As designed the return air are at the bottom of the wall and the supply at the top. I have done air flow and balancing for over 40 years. my solution is to extend the return air on 2nd floor to the top of the wall and restrict the return air on 1st floor. And when I am home I move the fan switch from auto to on. No my second floor gets down to 2 to 3 degrees of 1 st floor.
I also have a programmable stat set to 78 for off hours and 75 for home hours.
A/C should not run continuously, any more than heat should.
If you are running your A/C 7/24, it is either undersized or failing (low refrigerant, dirty coils, blocked airflow).
An A/C that is too large will run 5 minutes every 1-2 hours - which doesn’t get much more than the air cool - you wand everything - rugs, furniture, even the sheet rock in the walls to be cool - and that requires more time than a massive blast of cold air which immediately trips the thermostat.
Adjust vents, give it 24 hours to see the changes. Repeat.
If in a hurry - shut off ALL downstairs vents and see ir the central unit can even cool the upstairs all by itself. Then start opening lower level vents (or get an aux. A/C).