Our newly purchased townhouse has two thermostats with one furnace & one A/C unit (the home inspector told us the thermostat in the downstairs is the main one). When we set both thermostats at 78 degrees, cold air is continuously forced out from the vents in the downstairs even with the temperature in the downstairs reaching 73 degree, while the upstairs is still at 82 degree.

Any suggestions/tips on operatures will be greatly appreciated (the previous owner did not leave any manual for us).

You will need to balance the AC system to get the airflow correct. In order to do that you may need to install dampers in the ducts, or at least one in the main ducts moving air to each floor. You may already have dampers installed but not adjusted. I cannot imagine doing an installation without them on a two-story home. There are also booster fans that you can buy in case you need more power to move air upstairs, but the first thing I would check is the inside coils on the AC unit. Most likely they are stopped up with dirt. You will have to look at the side from where air enters the coils because that's where you can see the dirt. Sometimes the coils are full of dirt and you cannot really see it because it's between the fins. The first thing you want to do before you do anything is clean the coils with alkaline coil cleaner. It foams the dirt out. You can wash the coils out with a water hose or get an electric pressure washer with a fan tip. On AC coils you use a maximum of 1500 PSI with a fan tip. If the AC unit is for a large house a water hose does not normally have enough pressure to get through the coils. When the unit is 2 1/2 tons and below you can use a water hose to wash the coils out; above that and you need a pressure washer. Make sure to clean out drain line, too. Make sure to wear rubber gloves and a mask and face shield. Breathing in the particles that a pressure washer stirs up causes a type of pneumonia that has to be treated with antibiotics and steroids and you don't want that. Make sure to clean and oil the blower, too. It only takes 1/16" of dirt in the cups of the blower to lose 33% of the airflow. The outside coils are easier because you only have to remove the top cover to wash out those coils with a water hose and coil cleaner. Once the coils are clean the airflow problem could be solved, but if not, you can balance the airflow. You cannot balance a dirty system