Staying cool takes a lot of power. A central air conditioner can use up to 6,000 watts to stay running throughout the day and 4-6 times that wattage to start up.
Now double that, and you'll realize how much power you would need to power not one, but two central air conditioners during a blackout.
However, if you have a large family and a large home, you probably have little choice but to keep both air conditioners running on a hot summer day, power outage or not.
Most modern automatic transfer switches come with built-in A/C shedding functionality. A/C shedding is the ability for a transfer switch to "shed" or turn off the air conditioner so other appliances can get power.
A/C shedding is crucial to being able to power two air conditioners. Since the amount of power it takes to start just one A/C unit is so great, the generator wouldn't be able to start both at once. With A/C shedding, the transfer switch prevents both generators from trying to start at once. Instead, it starts them one at a time while also balancing other power needs in the home.
The following sequence of events is an example of how A/C shedding can manage power loads:
If at any point during this process another home appliance, such as an electric water heater, needs to turn on but there is not enough power, A/C unit startup and operation will be delayed or turned off.
As you can see, short of buying an extremely large, whole-house generator, A/C shedding is critical to running two air conditioners, along with other appliances in your home.