Connect multiple RFID-RC522 cards?
Connect everyone to the MOSI / MISO / SCK and ground, then define every SDA all the way around. and set respectively high low depending on where you want to write.

How ever im not sure if you need to set high or low if you want to read. But since you got multiple you can play around with that your self. Feel free to add to this question with the result for future googlers.

#include &lt;SPI.h&gt;
#include &lt;MFRC522.h&gt;

#define RST_PIN           7          // Configurable, see typical pin layout above
#define RST_PIN2           8          // Configurable, see typical pin layout above
#define SS_PIN           9          // Configurable, see typical pin layout above
#define SS_PIN2          10          // Configurable, see typical pin layout above

....

MFRC522 mfrc522(SS_PIN, RST_PIN);   // Create MFRC522 instance.
MFRC522 mfrc522_2(SS_PIN2, RST_PIN2);   // Create MFRC522 instance.

....


void setup() {
  Serial.begin(9600); // Initialize serial communications with the PC
  SPI.begin();        // Init SPI bus
  mfrc522.PCD_Init(); // Init MFRC522 card
  mfrc522_2.PCD_Init(); // Init MFRC522 card