If there is no air flow, you have a frozen evaporator coil.

The cheap filters allow good airflow and changing it often allows the AC to not work as hard saving you from replacing an AC too soon.

I have done demo work on moldy crap houses. get a dumpster, gloves, and a respirator rated for that kind of work. you can buy a garden pump sprayer and mix a very light bleach/water mixture and spray areas you are working on, it will kill germs and most importantly keep the dust down so you don't breath it in. setup a negative air flow so nasty air gets sucked out and sanitize the place with bleach/antimicrobial treatment as you go and once you are done. you dont wana fail a mold test after the work is done!

Install 2 furnaces or add a zoning system which will give both people a thermostat, it uses dampers to close off the airflow to the upstairs or down.

Correct size, matching air handler and heat pump, proper airflow and ductwork.

This is done with air flowing through system for 15 minutes to 30 minutes to see if there is any leak.

You can put them on the inside at the bottom to reduce air flow into your unit. 

The home reaches 90 degrees inside with no airflow.  

This means making sure all rooms receive heat and airflow on a consistent basis.

The system has a diverter in the crawl space that will switch the air flow from the ductwork that delivers the air upstairs or to the ductwork that delivers the air downstairs.