Python to Arduino problem reading/sending data for Stepper motor control

I have a working Arduino sketch that accepts inputs from the arduino serial monitor and will move two stepper motors from position to position. It works well when controlled from the Arduino
Serial Monitor

// MultiStepper.pde

#include <AccelStepper.h>
#include <MultiStepper.h>

//define the steppers and pins used
AccelStepper stepper1(1, 9, 8);
AccelStepper stepper2(1, 7, 6);

//  MultiStepper
MultiStepper steppers;

int pos1=0; //absolute positions of each motor 
int pos2=0;

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

    // Configure each stepper
    stepper1.setMaxSpeed(400);
    stepper2.setMaxSpeed(400);

    // Then give them to MultiStepper to manage
    steppers.addStepper(stepper1);
    steppers.addStepper(stepper2);
    Serial.println("Enter relative positions for each axis followed by 'Enter'");

}

void loop() {
    long positions[2]; // Array of desired stepper positions
    // Positions are absolute. Origin is set at start-up.
    //So a move to say 200 will move 200 steps. If the next position read is
    //also 200 the stepper will not move. 


    while (Serial.available() == 0);
    positions[0] = Serial.parseInt(); //read int 
    while (Serial.available() == 0);
    positions[1] = Serial.parseInt(); //read int

    steppers.moveTo(positions);
    steppers.runSpeedToPosition(); // Blocks until all are in position

    pos1 = pos1 + positions[0];
    pos2 = pos2 + positions[1];
    
    Serial.println("Absolute positions are:");
    Serial.println(pos1);
    Serial.println(pos2);
}

I have tried to write a python program that will eventually read positions from a file and send them to the Arduino.

#!/usr/bin/python
#serialToArduino.py
#   my attempt to send stepper motor positions to an Arduino
#   it works for the first pair of positions but not the second???
#   when the comms are sorted I intend to read the positions from a text file

import serial

# open serial port
ser = serial.Serial('/dev/ttyUSB0')
# the arduino sketch invites 2 integers to be input via serial comms
msg = ser.readline().decode().strip()
print(msg) #            This is the "Enter..." string

ser.write(b'500') #     send 1st Xpos
ser.write(b'-500') #    send 1st Ypos

msg = ser.readline().decode().strip()
print(msg) #            This is the start of the report "Absolut...
msg = ser.readline().decode().strip()
print(msg) #            reporting the 1st Xpos
msg = ser.readline().decode().strip()
print(msg) #            reporting the 1st Ypos

# The steppers run to position here OK

#ser.flushInput();ser.flushOutput() this has no effect so commented out

ser.write(b'000') #  send a new  Xpos
ser.write(b'200') #  and new Ypos

print("here") #         debug: it prints the string then dies????
              #         this is as far as it gets.
msg = ser.readline().decode().strip()

print(msg) #            This is the 2nd report "Absolute..." string

msg = ser.readline().decode().strip()
print(msg) #            2nd Xpos (not attained)
msg = ser.readline().decode().strip()
print(msg) #            2nd ypos (not attained)

ser.close()             # close port

The system hangs at the marked position with the python shell showing:

======== RESTART: /home/trevorj/MyPrograms/Python/serialToArduino.py ========
Enter relative positions for each axis followed by ‘Enter’
Absolute positions are:
500
-500

Can someone please tell me what I am doing wrong.

Thanks
here

try now

void loop() {
    long positions[2]; // Array of desired stepper positions
    // Positions are absolute. Origin is set at start-up.
    //So a move to say 200 will move 200 steps. If the next position read is
    //also 200 the stepper will not move. 


    while (Serial.available() > 0) {
           positions[0] = Serial.parseInt(); //read int 
    }
    while (Serial.available() > 0) {
           positions[1] = Serial.parseInt(); //read int
    }
    steppers.moveTo(positions);
    steppers.runSpeedToPosition(); // Blocks until all are in position

    pos1 = pos1 + positions[0];
    pos2 = pos2 + positions[1];
    
    Serial.println("Absolute positions are:");
    Serial.println(pos1);
    Serial.println(pos2);
}

Thanks Surbyte but that's no help. If I change the lines from "==0" to ">0" the Arduno just keeps receiving zeros. the output of the serial monitor is:

Enter relative positions for each axis followed by 'Enter'
Absolute positions are:
0
0
Absolute positions are:
0
0
Absolute positions are:
0
0

and so on without me even touching a key.

If I try to run it under my python program the output is:
======== RESTART: /home/trevorj/MyPrograms/Python/serialToArduino.py ========
Enter relative positions for each axis followed by 'Enter'

and it hangs there. ????

Solved! Adding line return after each integer sent solved the problem:
ser.write(b'-200\r') # send a new Xpos
ser.write(b'200\r') # and new Ypos
don't know why the first positions were ok but now its all ok