We have two cats: littermates.
Generally, that means we have one cat outside, and one inside. It is as if they have coordinated a schedule. Often, they will meet in the doorway, one going out, the other coming in. They'll give each other a quick sniff and go their separate ways.
The way I see it, this can mean one of two things.
Either they cannot stand each other (which is possible, as I get the impression that Ellie is kind of a demanding jerk).
Or, they have taken it upon themselves to guard our perimeter against mice, songbirds, squirrels, koi, snakes, baby bunnies, mummies and other things that require killing, 24/7. This is also a strong possibility, as they both seem to take their jobs very seriously.
It's just one of those situations when I wish I could listen in on their little cat conversations.