Yes, that's right. In your case, you need the Cat mate super selective. There you can selectively set which cat is allowed in or out.

Yes, I have the same two cats the older one goes out and the younger one is an indoor cat. It was her choice. So yes as long as only the one you want to come in and go out has the collar with the tag on. He will be the only one the door will work for. I’ve had mine about a year and it worked great until the battery’s ran out so I changed them and only get a flashing screen. Reset is suppose to work if you press the + button for 30 seconds. That did not work. So now I’m asking here if anyone else has had that problem. My cat was out for 1/2a day in the Florida heat because he couldn’t get in. I didn’t realize until I got home. I had to apologize profusely because I felt so bad. I have now ordered the automatic magnet door. I’ll see how that works.

With this version you cannot determine which cat can go out, correct. There is another version (I think it's called Selective), but it doesn't work with the chip the cats already have from the vet, so you have to put a collar on them.