The Wiegand library was based on my code which originally supported three readers and is easily changeable to support any number of readers. It is here.
http://www.thebox.myzen.co.uk/Hardware/Crazy_People.html
However if the library is written correctly you should just be able to define an other instance of it.
WIEGAND wg1;
WIEGAND wg2;
WIEGAND wg3;
And so on. But if you can not change the PIN numbers then no you can not just parallel up readers unless only one is is being used at any one time and the readers have open collector outputs.