byte buff[250];

void setup() {
  Serial1.begin(9600);
  Serial2.begin(9600);
}

void loop() {
  if(Serial1.available()){
    Serial2.write(buff, Serial1.readBytes(buff, Serial1.available()));
  }
  if(Serial2.available()){
    Serial1.write(buff, Serial2.readBytes(buff, Serial2.available()));
  }
}