Multiple RFID readers with Arduino Mega

Hi everyone,

I am developing a contactless card system using Arduino to allow users to buy snacks at a canteen and order lunch.

I want to use 2 RFID readers simultaneously, however, I get a lot of errors when I try to do so. I followed this GitHub repository and adjusted the code to my needs but I get the following enormous list of errors when I compile the code:

/Users/alexoort/Documents/Arduino/Payment_system_2_cards/Payment_system_2_cards.ino: In function 'void setup()':
Payment_system_2_cards:122:12: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
     mfrc522[reader].PCD_Init(ssPins[reader], RST_PIN);
            ^
Payment_system_2_cards:126:12: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
     mfrc522[reader].PCD_DumpVersionToSerial();
            ^
/Users/alexoort/Documents/Arduino/Payment_system_2_cards/Payment_system_2_cards.ino: In function 'void loop()':
Payment_system_2_cards:154:16: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
   if (!(mfrc522[reader].PICC_IsNewCardPresent() && mfrc522[reader].PICC_ReadCardSerial())) {
                ^
Payment_system_2_cards:154:59: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
   if (!(mfrc522[reader].PICC_IsNewCardPresent() && mfrc522[reader].PICC_ReadCardSerial())) {
                                                           ^
Payment_system_2_cards:162:31: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
   for (byte i = 0; i < mfrc522[reader].uid.size; i++)
                               ^
Payment_system_2_cards:164:26: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
      Serial.print(mfrc522[reader].uid.uidByte[i] < 0x10 ? " 0" : " ");
                          ^
Payment_system_2_cards:165:26: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
      Serial.print(mfrc522[reader].uid.uidByte[i], HEX);
                          ^
Payment_system_2_cards:166:35: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
      content.concat(String(mfrc522[reader].uid.uidByte[i] < 0x10 ? " 0" : " "));
                                   ^
Payment_system_2_cards:167:35: error: no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')
      content.concat(String(mfrc522[reader].uid.uidByte[i], HEX));
                                   ^
Multiple libraries were found for "LiquidCrystal_I2C.h"
 Used: /Users/alexoort/Documents/Arduino/libraries/LiquidCrystal_I2C-master
 Not used: /Users/alexoort/Documents/Arduino/libraries/NewliquidCrystal
exit status 1
no match for 'operator[]' (operand types are 'MFRC522' and 'uint8_t {aka unsigned char}')

Is it possible to use multiple RFID readers with Arduino or am I doing something wrong?

Thank you in advance,
Alex

p.s. the code is attached because it is too long to put between code tags.

Payment_system_2_cards.ino (22.7 KB)