1. Use a dual tuner receiver. These receivers have two tuners, so you can connect both an XM and a Sirius antenna. You can then select which service you want to listen to by pressing a button on the receiver.
2. Use a splitter. A splitter is a device that takes the signal from one antenna and splits it into two signals. You can then connect one signal to an XM receiver and the other signal to a Sirius receiver.
3. Use an FM modulator. An FM modulator converts the digital signal from an XM or Sirius receiver into an FM signal. You can then connect the FM modulator to your car's FM antenna input. This will allow you to listen to XM or Sirius through your car's FM radio.
If you're not sure which method is right for you, you can consult with a car audio professional. They can help you choose the best way to connect XM and Sirius to your stereo system.