Poms herding behavior causes problem for Chihuahua

I have an 8-month Pom (Ally) and a 4-yr old 5lb Chihuahua (Cody). When I take the dogs outside, Ally, who is on a tie out leash, will hunch down into the grass like a cat waiting to pounce on its' prey. When Cody approaches to come back into the house, Ally herds him and tries to stop him from coming onto the deck and into the house. I have to physically hold her back as she fights to get at Cody. I've tried to let them work it out on their own, but due to Cody's small size, Ally will sometimes plow right over him. Once they get into the house they're fine and they play together, clearly still trying to show one another who is top dog. I appreciate any advice on how to stop this behavior.