The 4 tap Set button test is used to verify the Access Points are in range of each other and on opposite 120v legs. Tap the Set button on one of the Access Points 4 times. Look at the LED on the other Access Point. If consistently blinking it is hearing the test RF message being sent from the first Access Point. If blinking Red the second Access Point is on the same 120v leg as the first Access Point, not the desired result. The second Access Point should be blinking Green indicating it is hearing the first Access Point and they are on opposite 120V legs. This test does not configure the Access Points as they have nothing to configure. The 4 tap Set button test simply verifies if the second Access Point is installed on the opposite 120v leg.
There is nothing that needs to be done to add additional Access Points. The same 4 tap Set button test can be run to see if the new Access Point is in RF range and what 120v leg it is plugged into.