Hello Sir,
I am using Arduino Mega and as far as shield concern, it is L293D (L293D Arduino Dual Motor Shield – Art of Circuits).
Of course I have tried that, but I saw no pattern. It seems that the motor shield gives the according Voltage to the first motor, -0.1 to the second motor and less voltage to the rest of the motors. I am not even sure of what I am saying, I just observe the whole project.