Controlling stepper motors via serial monitor - AccelStepper library

Cheers for the guidance, have been trying today to implement your suggestions and this is what I have changed it to.

#include <AccelStepper.h>

AccelStepper stepper1(AccelStepper::FULL4WIRE, 5, 6, 7, 8); //Sets up the pins for stepper one
AccelStepper stepper2(AccelStepper::FULL4WIRE, 9, 10, 11, 12); //Sets up the pins for stepper two

int start_var = 0;
int movement = 360;
long newStepper1Pos = 0; // varialbe for the new stepper position for variable 1
long newStepper2Pos = 0; // variable for the new stepper position for variable 2

void setup()
{  
    
    Serial.begin(9600);
    
    stepper1.setMaxSpeed(200.0); //Sets the max speed for stepper 1
    stepper1.setAcceleration(100.0); //Sets the acceleration for stepper 1
    stepper1.moveTo(movement);
    
    stepper2.setMaxSpeed(200.0); //Sets the max speed for stepper 2
    stepper2.setAcceleration(100.0); //Sets the acceleration for stepper 2
    stepper2.moveTo(movement);

}
   
    char inBuffer = "";

void loop()
{
      
    if(Serial.available()>0) // If any information is avaiable coming from the serial monitor
    {
      inBuffer = Serial.read(); // Puts the information coming from the serial monitor in the input buffer
    }
       
    if (inBuffer == 'a') // If the command sent is an 'a'
    {
      Serial.println("test point 1"); //Serial print for test point 1
      
      if (stepper1.distanceToGo() == 0)
      newStepper1Pos = -stepper1.currentPosition;
      stepper1.moveTo(newStepperPos);
      
      if (stepper2.distanceToGo() == 0)
      newStepper2Pos = -stepper2.currentPosition;
      stepper2.moveTo(newStepperPos);
    }

    stepper1.run();
    stepper2.run();
    inBuffer = ""; //Sets the input buffer back to nothing as to not create a loop

}

I am getting an interesting error message in the form of "wrong type argument to unary minus" which I don't really know what it means.

I tried setting the moveTo() equal to zero so that maybe when the program moved, it wouldn't move until a command was sent, however this just resulted in no movement what so ever.

    stepper1.setMaxSpeed(200.0); //Sets the max speed for stepper 1
    stepper1.setAcceleration(100.0); //Sets the acceleration for stepper 1
    stepper1.moveTo(0);
    
    stepper2.setMaxSpeed(200.0); //Sets the max speed for stepper 2
    stepper2.setAcceleration(100.0); //Sets the acceleration for stepper 2
    stepper2.moveTo(0);

I am looking through the functions that come with this library to see if there is a way of just setting an initial start point for the servo motors so that they can always start from the same position. Thanks again for the help!