Can I upsize my air conditioner if I have a two-speed compressor?
During the AC season close off outlets that go to the cooler area of your house, and be sure the other outlets are wide open. Put in a brand new, low restriction furnace air filter. Keep curtains closed all day on the sunny side of the house.
After these steps you may use a portable on the lower floor if you need it. As long as the portable unit has 2 air ducts going outside, it will have no adverse effect on your central system.
Also, be sure to have the central AC set so that it comes on well before noon. If you wait until it feels like it's getting too warm ... then the system will never catch up.