DIY RFID reader antenna array
It might be simpler just to build one antenna with the requisite operating area than trying to multiplex several smaller ones. The PN532 uses a magnetic field to talk to RFID devices and the antenna it uses is a coil of wire basically resonant tuned to emit more magnetic field than could be got for the same power supply current as an untuned antenna: -



Circled in red are the resonant tuning components. An inductance of 560nH and 220pF on their own tune the output to be a little above 14 MHz and the loop, C and C2 bring this down to 13.65 MHz.

I don't have details on the antenna inductance but I suspect that a single turn of wire probably about 0.5m diameter might be tunable and save you all the headaches of trying to multiplex multiple coils - because they are resonant they won't appreciate too much added series resistance and the voltage may be several tens of volts p-p.

Here is AN1445 from NXP that desribes how to design the antenna.