That code is for a Wiegand reader, the way he dose it you can not expand the number of readers because there are not enough direct interrupt pins on the Arduino.
So you have to use the pin change interrupt which is a bit more complicated.
This project has three Wiegand readers connected to it, look at the code and see how it is done.
http://www.thebox.myzen.co.uk/Hardware/Crazy_People.html