in theory, you could take apart your 2nd fob (or get a 3rd one and do it), and just take the metal key part out... drill a little hole in the back of the key (if there isn't one there already from the swivel) for a key ring if you need to...
I'm not sure if there is a "door only" key available. or, if you have onstar, just leave your keys locked inside, and call them when you are done and ask them to unlock
