You can use many instances of NSS in parallel as long as there is not too much traffic as the Arduino is single threading.
But, only one instance at a time can receive. NewSoftSerial uses an interrupt to catch the incoming data. When the interrupt fires, the data that is incoming is for the active instance.
Switching which instance is active occurs whenever a method on any instance is called. Changing the active instance changes which interrupt handler is attached, which changes which instance can receive data.
Multiple instances can send easily enough, but only one instance can receive at a time.
It looks like you think you can receive from two instances at once, which is not the case. To do that, you need two (or more) hardware serial ports, which use two (or more) interrupts - one for each hardware port. The only Arduino with multiple hardware serial ports is the Mega.