with xm you can just remove the unit and place it in your home receiver (if you have one) b/c the home device has the antenna... you can only put the unit in another car if you hook up an antenna there too..

if you want to operate two separate units you have to have two separate services but with xm its only about 5 bucks to add another plan.. i think xm runs about 12-15 bucks a month but drew paid for the entire year so i don't remember