Do you have a stand alone Tivo and a regular D* receiver or do you have a DirecTivo? If you have the SA Tivo, you need to run another line from your switch (like Ray H said) or directly from your dish to another receiver. Same thing with the D-Tivo except you just connect the new line to the second tuner.
Running the cable can be tough if you have the dish on the roof and receivers in the basement. What's your situation?
-Robert