I just finished a project that uses two mifare 13.56 mhz RFID readers, and figured this might come in handy for somone.
Because these RFID readers are SPI devices, they can be connected to the same two data pins (MISO & MOSI), and clock pin (SCK), and only two pins (SDA & RST) have to be unique. This makes a total of 7 data pins, plus 3.3v and Gnd. I'm taking the 5 sets of serial numbers, and adding them together, and making each reader accept a different set of numbers. Run the serial monitor, scan your badge or keyfob, and then enter the total code into your sketch to make it active. You could have two security doors, and one badge will get you through one, but a different badge can get you through both.
Code & Library -