Hello there,
To begin, I'd like to precise I am quite unexperienced in Arduino and electronics so far, I just want to help a colleague for his graduation project. 
So first, here is the big idea :
My friend want to conceive a kind of a "board" with 6 slots. Each slot would reckonize a signal and these six devices would return a sequence.
To illustrate this quickly :
6 slots : [ A ] --- [ B ] --- [ C ] --- [ D ] --- [ E ] --- [ F ]
|||||_____|
||
Arduino
So we thought we could use NFC tags with NFC receivers as these slots. So in theory, it doesn't seem like a bad idea, since he already bought a ATmega2560, to connect some devices to this master.
But since then, it was becoming really unclear for me...
Since he was running out of time for his project, I thought quickly that NFC Shield (PN532) would do the job perfectly. We took 6 of them and tried to connect them. Then, I studied the Adafruit NFCShield I2C library to make all this stuff work.
And indeed, I was able to read the NFC tag through one shield, but when I used 2 of them, only the value of the first tag was returned.
We found out we had to reassign the "id" of each shield. But what I read in Adafruit FAQ about their shields that is it was not possible to use multiple shields on one Arduino, since it had a fixed address. At this point, I'm really upset about it, since we have to collect the tags in 6 separate devices for the needs of my friend installation !
I feel bad about that, since I was convinced I was able to help my friend. Maybe I missed an important point, and I'm searching for a solution to read 6 tags at the same time, on 6 receivers, even if shields would end up unused (though it would be a pain to restart all from the start now).
Thank you for your solutions, I am very sorry if I was not very clear, I have a few knowledge of electronics... ^^'