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
I placed an aluminum sheet (with epoxy) the size of the panel andagainst the removable panel to act as a ground plane for the antennas and then attached the antennas one near each end of the sheet say at the 1/4 way in from the edges. My signal are maxed out on the XM radio and the GPS works great but no signal strength meter availiable as I have the ALL TALKING VCommand unit.