Pb with 2 arduino connection mySerial.write

hey i have problem to pilot second card with myserial.write

i need to pilot led on 2nd card if led on 1st card are open like circulation color stop
but i arrive to send text to second card but when i send other text they are always the part of 1st text with him ?
can you help me please ?

// C++ code
// programme esclave

#include <SoftwareSerial.h>
String readString;
SoftwareSerial mySerial(10,11); //rx tx

int feurouge2 = 8;
int feuvert2 = 7;

void setup()
{
  mySerial.begin(9600);
 Serial.begin(57600); //vitesse de transmission baud
  Serial.println("demarrage de Programme");
  pinMode(feurouge2 , OUTPUT);
  pinMode(feuvert2 , OUTPUT);
}

void loop()
{

  
  while(mySerial.available()){
    delay(10);
    char c = mySerial.read();
      readString += c;
  }
  
  Serial.println(readString);
  
  if(readString.indexOf("F")!=-1)
  {
    digitalWrite(feurouge2,LOW);
    digitalWrite(feuvert2,HIGH);
  } delay(10);
  if(readString.indexOf("O")!=-1)
  {
    digitalWrite(feurouge2,HIGH);
    digitalWrite(feuvert2,LOW);
  } delay(10);
  
 
}
// C++ code
// programme maitre
#include <SoftwareSerial.h>
String readString;
SoftwareSerial mySerial(10,11); //rx tx

// déclaration de l'etat des LED
int EtatLedV;
int EtatLedR;

//sortie des LED
int feurouge1 = 8;
int feuvert1 = 7;
  



void setup()
{
  mySerial.begin(9600);
 Serial.begin(57600); //vitesse de transmission baud
  pinMode(feurouge1 , OUTPUT);
  pinMode(feuvert1 , OUTPUT);
  Serial.println("demarrage Programme Feu chantier");
  //lecture de l'etat des sorties
  
   
  
}

void loop()
{
 
  digitalWrite(feurouge1,HIGH);
  digitalWrite(feuvert1,LOW);
  for (int i=0; i <= 20; i++) 
  {
  mySerial.write("FFFFFF\n"); //envoi de la chaine de caractère 
    Serial.println("envoi F");
  }
  delay(1000);
   for (int i=0; i<= 200; i++)
  {
  mySerial.write("OOOOO\n"); //envoi de la chaine de caractère 
  Serial.println("envoi O");
  }
  
  digitalWrite(feurouge1,LOW);
  digitalWrite(feuvert1,HIGH);
  delay(1000);
 
}

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