Code

/*
Connection for Uno or other 328-based platforms:
  left wheel encoder (DO)  -> Digital pin 2
  right wheel encoder (DO) -> Digital pin 3
*/

#define LEFT 0
#define RIGHT 1

long coder[2] = {0,0};
int lastSpeed[2] = {0,0};
void setup(){
Serial.begin(9600);
attachInterrupt(LEFT, LwheelSpeed, CHANGE);
attachInterrupt(RIGHT, RwheelSpeed, CHANGE);
}

void loop(){
static unsigned long timer = 0;
if(millis() - timer > 100){
Serial.print("Coder value: ");
Serial.print(coder[LEFT]);
Serial.print("[Left Wheel] ");
Serial.print(coder[RIGHT]);
Serial.println("[Right Wheel]");

lastSpeed[LEFT] = coder[LEFT];
lastSpeed[RIGHT] = coder[RIGHT];
coder[LEFT] = 0;
coder[RIGHT] = 0;
timer = millis();
}
}

void LwheelSpeed(){
coder[LEFT] ++;
}

void RwheelSpeed(){
coder[RIGHT] ++;
}