SoftwareSerial issue in Arduino Diecilima

I have interfaced a sim900a module and gps module to my arduino via Software Serial. Only one works at a time. If i start both of them at 9600 baudrate, they stop responding. Please help? What baudrate should i keep?

Hi, welcome to the forum.

The SoftwareSerial listens only to the active port, that is one port at a time.

Select the port to listen to with listen().
It is done like this: Talk to a serial device and get the answer until that is finished. Listen to other port and talk to the other serial device until that is finished. And so on.
So you can create two or more software serial ports with baudrates of 9600 or higher, unless there are many other interrupts.

The AltSoftSerial is for just one serial port : AltSoftSerial Library, for an extra serial port