Make sure the serial monitor speed in the dropdown box matches the one you specify in the begin() call of Serial or NewSoftSerial.
Also, remember that the Arduino pins RX and TX are shared with the FTDI chip (i.e. the USB serial port), so if you use those to make the two arduinos talk to each other there might be problems (haven't tried myself though).
I suggest you leave the hw serial (i.e. pins TX and RX) alone and use two other pins with the software serial lib.
HTH