Initially, she hung out by the back door and in her crate. But familiarity breeds bravery and she's starting to expand her horizons! This of course makes Tessa crazy. It is in her eyes, her house. This darned interloper had better stay in their place! Tessa has taken to perching on the landing, keeping a close eye down the stairs to ensure Reba doesn't encroach the upstairs territory. Or lying across the doorway to make sure that darned dog doesn't enter the room where I am. While kind of funny, this is actually bad behavior. You see, I'm the alpha dog and I say who goes where - not Tessa and not Reba. For the most part I prefer Reba to stay in her area of the back door and the neighboring room where her crate stays, but it's not Tessa's job to monitor this. (Reba's not housebroke - knock on wood - so far no accidents!)
Reba is actually worse than Tessa because she's extremely territorial and protective of me. She has the audacity to growl at Tessa when she comes to the door area. Whereas Tessa knows this is bad behavior and responds to a verbal command, Reba needs a strong negative reinforcement to stop the behavior. (I love Caesar and the Dog Whisperer!) I'm curious to see if they will ever be able to deal with both being in the same room as I am.
However, it's good training and socialization for them to work through this transition stage.