SoftwareSerial can only receive data from one port at a time. By default, the port is the one that had begin() called last.
In your case, that is why pin 8 works.
It is possible to switch which port you listen on, by adding nssi1.listen() and nssi2.listen() to your program.
However, the problem is knowing which reader is going to be used. If you are listening on nssi1 and the user swipes on nssi2, the data will be lost, I think.
There may be a way around this, but I'm not familiar with RFID readers.