Why can't I control my external source device from my second HDTV?

This is usually a matter of sensor alignment. The IR sensor with the GREEN tip goes into your Rx and should be in the open so the beam from the cable box remote can see the sensor. If your TV has a shiny

The IR sensor with the GREEN tip goes into your Rx and should be in the open so the beam from the cable box remote can see the sensor. If your TV has a shiny reflective frame, try not connecting it to the frame if you seem to have a limited range.

The IR Blaster with the BLACK tip goes into your Tx. This should be on the front of the cable box's bezel, not on top of it. Moving the sensor slowly from the right across the front, while someone else uses the remote to bring up Guide or Change Channel. As soon as the box responds, place tape on the cable box to mark the spot. Remove the sticky on the Velcro base, connect to the front of the cable box and place the sensor on this so the little diode is facing the IR eye in the cable box.