Arduino Mega 2560 : Tx and Rx 1,2,3 do not work?

i am a noob, so be gently, please.

i am sure i attached the arduino with 2 driver correctly, and adjust the code accordingly. the drivers and motors arent broken. the tx and rx 0 works, but tx1 and rx2 doesnt work. am i missing something?

void setup()
{
  Serial.begin(9600);
  Serial1.begin(9600);
 }

void loop()
{
  Serial.print('O');  // turn on stepper motor
  Serial.print('S');  // set speed
  Serial.write(10);   // to 10

  Serial.print('<');  // rotate counterclockwise
  Serial.print('M');  // set micro-stepping to
  Serial.write(1);    // 1
  Serial.print('G');  // run the stepper motor
  delay(3000);
  
  Serial1.print('O');  // turn on stepper motor
  Serial1.print('S');  // set speed
  Serial1.write(10);   // to 10

  Serial1.print('<');  // rotate counterclockwise
  Serial1.print('M');  // set micro-stepping to
  Serial1.write(1);    // 1
  Serial1.print('G');  // run the stepper motor
  delay(3000);
  
}

after i upload this code, only motor that connect to Driver 1 moves. whats the problem?

Motors may need current that your mega cannot provide.Use an external power source for the motors That happens if you swap the boards ?

i did use an external power supply. 12v, 2A adapter. i have another adapter with 12v, 3.75A. should i use that instead? instruction of the motor driver states that the current supplied should not exceed 2 A. i know that the drivers are connected in parallel, which means the current is devided by 2. but i kinda scared to do that, because i dont have the spare or extra money to buy another driver, if this cuase the driver to be broken.

the problem thou, even if i just connect the driver and the motor and the motor driver to JUST Tx1 and RX1, the motor still wont move.