Serial Commmunication between two arduino
The problem is that a serial connection usually connects two devices, in your case it's three (sensor arduino, receiving arduino and your pc).

To get around this you have to use an additional software serial on different pins on the receiving arduino (unfortunately the atmega328 has only one hardware serial interface).
As long as you don't have to send anything from pc to the receiving arduino and from the receiving arduino to the sensing one you could try to simply disconnect the wire receiving-TX to sensing-RX and see if it works. In this latter case you could actually get rid of the receiving arduino at all.

Edit: another important point was mentioned in JRobert's answer: grounds of devices communicating with each other must always be connected!