Sabertooth 2x12 not working

Hi!

I have been tasked with trying to create a controller for an underwater ROV. I am just in the beginning stages of my project, but I will be using 2 RS-485 modules to make a master-slave relationship between an arduino uno and arduino mega (uno = master, mega = slave).

Right now i am just trying to get 2 motors to work at the same time with a sabertooth 2x12 motor controller. When i have both motors connected to the motor controller, only the motor connected to M1A & M1B will work. But when i just have one motor connected to M2B & M2A it will work without changing the code or anything else. Do you have any suggestions on how I could go about fixing this?

Power Supply → DPS-712M Regulated DC Power Supply
Motors → 750 GPH 12V >Johnson Pump

Connections: (right now im just using the arduino mega)
GND → 0V on motor controller
Vin → 5V
TX → S1

#define SBT_MOTOR1_FULL_FORWARD 127
#define SBT_MOTOR1_FULL_REVERSE 1
 
#define SBT_MOTOR2_FULL_FORWARD 255
#define SBT_MOTOR2_FULL_REVERSE 128
 
//shut down both motors
#define SBT_ALL_STOP  0
 
void setup() {
  Serial.begin(9600);
  killMotors();
}
 
void loop() {
  fastForward();//move fast forward for 3 seconds
  delay(3000);
 
  killMotors();
 
  fastReverse(); //move fast reverse for 3 seconds
  delay(3000);
 
  killMotors();
 
  turnLeft(); //turn left for one second
  delay(1000);
 
  turnRight(); //turn right for one second
  delay(1000);
 
  killMotors();
}
 
 void fastForward(){  //motors fast forward
    Serial.write(SBT_MOTOR1_FULL_FORWARD);  
    Serial.write(SBT_MOTOR2_FULL_FORWARD);
    Serial.println("motors fast forward");
  }
 
 
 void fastReverse(){  //motors fast reverse
    Serial.write(SBT_MOTOR1_FULL_REVERSE);  
    Serial.write(SBT_MOTOR2_FULL_REVERSE);
    Serial.println("motors fast reverse");
  }
 
  void turnLeft(){  //motor 1 full reverse and motor 2 full forward to turn left
    Serial.write(SBT_MOTOR1_FULL_REVERSE);  
    Serial.write(SBT_MOTOR2_FULL_FORWARD);
    Serial.println("motor 1 full reverse and motor 2 full forward to turn left");
  }  
 
  void turnRight(){  //motor 1 full forward and motor 2 full reverse to turn right
    Serial.write(SBT_MOTOR1_FULL_FORWARD);  
    Serial.write(SBT_MOTOR2_FULL_REVERSE);
    Serial.println("motor 1 full forward and motor 2 full reverse to turn right");
  }
 
  void killMotors(){
    Serial.write(SBT_ALL_STOP);   //kill motors for 0.5 second
    Serial.println("kill motors for half a second");
    delay(500);