I would suspission you would need to determine if the XM is on the same frequencies as the GPS, Plus the relative signal strength to be sure that the amps of the receiving sections are not goining to trip the AGC of one or the other, Next you would probably need a "Splitter" to keep the impedance oK for both receivers and antenna loading. Finally ther may be some other problem.
I have GPS and XM and decided to put both antenna in the hidden installation not much trouble. I did think of the common antenna and decided it was easier to install the second antenna. GRIP
