Who says you can't train cats?
What do you when you have two cats that do not get along? You keep them apart. Just over year ago we brought another cat into our family and we didn't ask the other cat if this was okay...we just did it.
We thought the two would get along--silly us. So we adapt, we persevere, we survive. When the outdoor cat wants to come in, we put the other cat behind a door. The problem is, when someone is outside, they don't always know if the indoor cat is behind a door so they're not sure if it's okay to open the door or not. We used to open the door just a crack and yell to those inside. My kids came up with a simpler method.
They knock two knocks on the door, then two more.
I thought this to be a pretty good plan, but what impressed me the most is when the indoor cat ran into a room as soon as she heard the knocks. It's pretty amazing. Many of you may be saying, "Of course, you can train cats." Apparently, that's true.