In response, we have been watching this thread, but lets consider some key facts. This summer has been the hottest on record in many years for many parts of the country. It is a rare day when the high temperature in Wichita, KS is higher than Miami, FL for a consistent number of days. This summer has been VERY hot. I know first hand, when my camping trip the first week of July - the temperature did not go below 100 degrees, in Pennsylvania.
Second, our air conditioner suppliers verify that a single 15K A/C should lower the inside temperature from ambient temperature 20 degrees. This process can take several hours if it is an extraordinarly hot day. The best practice is to lower your ambient temperature as early as possible, like early in the morning. This gives equiptment a better "head start". In addition, doors and windows remain closed. Shades drawn.
A second A/C will not lower the temperature any more, it will just lower it faster. With more air flow, you increase your efficiency.
This all being said, make sure air flow is checked in your unit. Pleniums, ducts, and other airflow devices have and will come loose. Please be sure to check and recheck these because there is nothing worse than being uncomfortable.
I hope this helps. As usual, there are unique circumstances that we cannot address individually. We rely on our dealer network and Heartland service department to look into specific instances. Please use these resources to make sure this is a comfortable summer.