I think your schematic might be wrong there. Only one MEGA is connected to a positive side of a battery.
The MEGA can only run on 7V to 12V. BUT PLEASE DO NOT apply the voltage to the
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
And also, I would only put in another MEGA when you don't have enough I/O pins.
Digital I/O Pins 54 (of which 15 provide PWM output)
In theory you could use 13 steppers on one MEGA. Try slowly scaling up.
You don't need PWM for steppers, maybe you're confused with servo's?
And why not use one of the stepper-driving MEGA's as the master? (In the first picture, slave 1 isn't even connected to the SDA/SCL lines of the I2C bus.