Multiple sensors connected to Arduino MEGA 2560 Serial ports
Your serial buffers and moving in and out of phase, which is distorting your display.

Try something like:

void setup() {
  String sensor1;
  String sensor2;
  Serial.begin(57600);
  Serial1.begin(9600);
  Serial2.begin(9600);
  while (!Serial) 
  {
  }
}
void loop() { // run over and over
  if (Serial1.available()) {
    sensor1 = Serial1.readStringUntil('\n'));
  }

  if (Serial2.available()) {
    sensor2 = Serial2.readStringUntil('\n'));
  }

  Serial.print(sensor1);
  Serial.print(sensor2);
  Serial.println();
  delay(1000);
}


I would also recommend the method suggested by @Majenko in his comment below your question.