Anyone know if such a thing exists? I'd rather have just one antenna on the outside of my car instead of two.

no

no

Nope, two different frequencies. For a patch antenna design, as are used in XM and GPS, they are designed for a single frequency.

I suggest you mount the GPS on your dash. I have it that way in two cars and it works fine. I haven't tried that with XM but suspect it will not work. There are 12 or more GPS satellites so I think the antenna always has a good line of sight from the dash.

There's 24 though only 12 are over the states at any given time.

I remember reading one person was able to combined the 2 antenna together using a "Y" gps adapter. Basically two signal antenna from the p-10xmt and the navigation unit to read from the adapter which access from one antenna. Notice the xm antenna and the gps antenna uses 2 different port size. You will need 2 extended gps wire to the adapter and one gps antenna. I still yet to confirm this by finding where I have read this thread. He had pictures of the combine unit and install.

yea no can do, my xm is on the dash and works fine.

Grab yourself a GM Sharkfin antenna. Put your gps and XM in it just like GM does. One antenna with 2 recievers in it.