I am looking for connecting to ADAFRUIT RFID PN532 readers to one arduino, I think its possible since I2C was for that, but how can i modify the code (Which is already in RFID ADAFRUIT LIBRARY) two access through the both readers at one instance ?
I am not familiar with the device or the library but the normal thing to do would be to eliminate any hardware problems such as conflicting use of pins, ensure that both I2C devices have a different address and to create 2 separate instances of the device using the standard library commands. Why do you want to create a single instance representing both readers ?
You can run the shield off SPI as well as I2C. However there are a few pins that you will need to change, like the tag present signal. Also mechanically the shield fits over the Arduino so you are going to have to hack it to add a second one, they won't stack. Also RFID readers should be seprated by about a meter to reduce mutual interference.
They have not put up the schematics yet so I can't tell if there is a way of changing the I2C address. If ther is not then you will have to multiplex the bus or have a software I2C bus which is again more hacking.