Other things won't get too far with a dog on the other side and you can lock doggy doors so things can't go through it. She may just be tired of being an outside dog and she knows where the real action is...inside with you. You are her pack leader and you live in the house, so the natural thing for a dog is to want to be where the rest of the pack is. I rescue dogs that have lived their whole lives as outside dogs, but in my house they live inside. Once they've had a taste of the inside life, they don't want to go back. (That sounds like what she's going through.) It's outside for potty and play, then straight to the door scratching to come back in.
Even though she has another dog with her she can still get bored and lonely because YOU (the pack leader) are the one she wants to be with, to protect, to cuddle. If you don't want to install a doggy door, you could just make her an inside dog, crating her while you're gone (that's if she's not housebroken and chews stuff) and letting her outside for her play time.