The pin AD0 sets the MPU-6050 at I2C address 0x68 or 0x69.
It turns out that this is not done at power up, but AD0 can change the I2C address at any time to 0x68 or 0x69.
If all AD0 pins of all MPU-6050 chips are set HIGH (3.3V), then all the MPU-6050 chips are at I2C address 0x69.
That I2C address of 0x69 is not used at all.
The AD0 of only one MPU-6050 is turned low, and the sketch communicates via de I2C bus to I2C address 0x68.
After that, the AD0 is set HIGH, and an other MPU-6050 is selected by setting it at address 0x68.