Also, with all the "new" keyless entry systems where you don't even have to push a button on the keyfob itself to unlock / start the car as long as one of the keyfobs is "in the vicinity", you'll have to worry about where to put the "extra" keyfob if you've separated them out. You could be leaving yourself open to theft from / and of the car itself if you leave the "other" keyfob in the car / trunk etc. The Chrysler 300 for one of the "common" rental cars has this feature.

Come to think of it, if a car is equipped that way and you only get one keyfob, you probably should test out the unlocking / starting of the car when you have the keyfob pretty far away from the car (in case they left the keyfob in the trunk - like mentioned sometime back here in the forum).