@arideba, is this regarding using two MRC522 units on the Spark or on an Arduino? Either way, the way the library works is that you create a reader "instance" by using its constructor like this in the examples:
MFRC522 mfrc522(SS_PIN, RST_PIN); // Create MFRC522 instance
Notice that you pass two values - the SS or chipselect pin and the RST or reset for the MRC522 board
So, to use a second board, you would create another instance and specify a different name (mfrc522 in the example above) and different SS and RST pins (so you need two extra pins) like this for example:
MFRC522 RFID1(SS1_PIN, RST1_PIN); // Create MFRC522 instance
MFRC522 RFID2(SS2_PIN, RST2_PIN); // Create MFRC522 instance
Of course, you need to define SS1_PIN, SS2_PIN, RST1_PIN and RST2_PIN in your code. The SPI connections (MOSI, MISO, SCLK) stay the same and goes to both boards. Then in your code you refer to each reader separately.
Please note that I removed the other topic you created with the same question.