Dear Marc
Jiarong is right. Out goes for all, in only for the chipped cat. However, we did not receive a collar tag with the delivery, which is also supposed to work. Besides, I don't think a collar is ideal, especially for a young cat. She could get stuck with it when climbing....
Maybe it's easiest if you chip her as well.
Have fun with your cats

All cats can get out, but only the chipped cat can get in. The cat cap also comes with a collar tag that works just like a chip.