Conversion byte() question

// Now spin both!
driveArdumoto(MOTOR_A, CW, 255); // Motor A at max speed.
driveArdumoto(MOTOR_B, CW, 255); // Motor B at max speed.

MOTOR_A and MOTOR_B store to "byte motor"
How is it reading this? I am trying to figure out how motor would be equal to both 0 and 1 at the same time. I am aware that that 12 decimal is =1100.

Is it loading a 0 to the first digit and 1 to the second?

areas in question.. Note this is not functional unless looked at full code.

 if (motor == MOTOR_A)
  {
    digitalWrite(DIRA, dir);
    analogWrite(PWMA, spd);
  }
  else if (motor == MOTOR_B)
  {

Full code

/* Ardumoto Example Sketch
  by: Jim Lindblom
  date: November 8, 2013
  license: Public domain. Please use, reuse, and modify this 
  sketch!

  Three useful functions are defined:
    setupArdumoto() -- Setup the Ardumoto Shield pins
    driveArdumoto([motor], [direction], [speed]) -- Drive [motor] 
      (0 for A, 1 for B) in [direction] (0 or 1) at a [speed]
      between 0 and 255. It will spin until told to stop.
    stopArdumoto([motor]) -- Stop driving [motor] (0 or 1).

  setupArdumoto() is called in the setup().
  The loop() demonstrates use of the motor driving functions.
*/

// Clockwise and counter-clockwise definitions.
// Depending on how you wired your motors, you may need to swap.
#define CW  0
#define CCW 1

// Motor definitions to make life easier:
#define MOTOR_A 0
#define MOTOR_B 1

// Pin Assignments //
// Don't change these! These pins are statically defined by shield layout
const byte PWMA = 3;  // PWM control (speed) for motor A
const byte PWMB = 11; // PWM control (speed) for motor B
const byte DIRA = 12; // Direction control for motor A
const byte DIRB = 13; // Direction control for motor B

void setup()
{
  setupArdumoto(); // Set all pins as outputs
}

void loop()
{
  // Drive motor A (and only motor A) at various speeds, then stop.
  driveArdumoto(MOTOR_A, CCW, 255); // Set motor A to CCW at max
  delay(1000);  // Motor A will spin as set for 1 second
  driveArdumoto(MOTOR_A, CW, 127);  // Set motor A to CW at half
  delay(1000);  // Motor A will keep trucking for 1 second 
  stopArdumoto(MOTOR_A);  // STOP motor A 

  // Drive motor B (and only motor B) at various speeds, then stop.
  driveArdumoto(MOTOR_B, CCW, 255); // Set motor B to CCW at max
  delay(1000);  // Motor B will spin as set for 1 second
  driveArdumoto(MOTOR_B, CW, 127);  // Set motor B to CW at half
  delay(1000);  // Motor B will keep trucking for 1 second
  stopArdumoto(MOTOR_B);  // STOP motor B 

  // Now spin both!
  driveArdumoto(MOTOR_A, CW, 255);  // Motor A at max speed.
  driveArdumoto(MOTOR_B, CW, 255);  // Motor B at max speed.
  delay(1000);  // Drive forward for a second
  // Now go backwards at half that speed:
  driveArdumoto(MOTOR_A, CCW, 127);  // Motor A at max speed.
  driveArdumoto(MOTOR_B, CCW, 127);  // Motor B at max speed.
}

// driveArdumoto drives 'motor' in 'dir' direction at 'spd' speed
void driveArdumoto(byte motor, byte dir, byte spd)
{
  if (motor == MOTOR_A)
  {
    digitalWrite(DIRA, dir);
    analogWrite(PWMA, spd);
  }
  else if (motor == MOTOR_B)
  {
    digitalWrite(DIRB, dir);
    analogWrite(PWMB, spd);
  }  
}

// stopArdumoto makes a motor stop
void stopArdumoto(byte motor)
{
  driveArdumoto(motor, 0, 0);
}

// setupArdumoto initialize all pins
void setupArdumoto()
{
  // All pins should be setup as outputs:
  pinMode(PWMA, OUTPUT);
  pinMode(PWMB, OUTPUT);
  pinMode(DIRA, OUTPUT);
  pinMode(DIRB, OUTPUT);

  // Initialize all pins as low:
  digitalWrite(PWMA, LOW);
  digitalWrite(PWMB, LOW);
  digitalWrite(DIRA, LOW);
  digitalWrite(DIRB, LOW);
}
// Now spin both!
  driveArdumoto(MOTOR_A, CW, 255);  // Motor A at max speed.
  driveArdumoto(MOTOR_B, CW, 255);  // Motor B at max speed.

both "MOTOR_A" and "MOTOR_B" look like the are stored in the value of "byte motor"

How is it knowing to spin both at the same time?

Now that I am thinking about it. It does not clear motor just sets

motor=0
motor=1 and runs it and there would be a few nanosecond delay between the starts of motor A and B

You have defined the motors here

// Motor definitions to make life easier:
#define MOTOR_A 0
#define MOTOR_B 1
// Now spin both!
  driveArdumoto(MOTOR_A, CW, 255);  // Motor A at max speed.
  driveArdumoto(MOTOR_B, CW, 255);  // Motor B at max speed.

The code 'instructs' motor_A (value 0 as defined above) to spin clockwise at full speed. Next it 'instructs' motor_B (value 1) to do the same.

When using #defines, the pre-compiler replaces all instances of MOTOR_A by the value that you specified. Same for MOTOR_B, CW and CCW.

Is it loading a 0 to the first digit and 1 to the second?

Not quite sure where 'digit' comes from? But after pre-compiling, your code is

// Now spin both!
  driveArdumoto(0, 0, 255);  // Motor A at max speed.
  driveArdumoto(1, 0, 255);  // Motor B at max speed.