Correct, on the KK2 the first motor output is where power for the board is supplied.
It is isolated from the rest of the motor / servo outputs.
No issue if all you are doing is supply a control signal to esc but if you need to power a servo then an additional power source is required.
I just use the esc for motor 1 to power the board and the esc for motor 2 to power the servo for the yaw assembly.
This was done because the KK2 is extremely sensitive to voltage dips at the ATMEL processor, therefore it was isolated from the rest of the power system with the exception of the receiver.
Same rule applies to additional power sources on the "servo" power rail side as other boards. Linear ESC supplies can be left hooked up parallel but not switching ESC's.
Thurmond