how do i tame my jack russell?

the aggressive JRT is dominating the other one, their is a few thing you could try

feed the dominant one first, ignore bad behavior, praise good behavior, play with them together make them tug a toy together, do not let the dominant one sit up on your lap.

when you notice any signs of aggression occurring scruff the dominant dog, this wont hurt him, wait until he is calm then let him go.

always pay attention to a calm dog.

Are they neutered? If not, have both dogs neutered. It will likely help. If it continues thereafter or they are already neutered, it would be a good idea to enlist the help of a qualified professional trainer or behaviorist who can observe your dogs' behavior, identify 'triggers', and suggest an appropriate and customized approach to your problem. Addressing aggression on your own often worsens the issue, especially when you address it by taking training suggestions from random strangers on the internet who may or may not have any idea what they are talking about.

JRT's are high energy dogs. They're designed to be working nonstop. In addition to the suggestions made by "oregano", make sure they have more opportunities for "structured play and exercise." That is, exercise on your terms and according to your rules. When they get bored, their ideas of fun tend to be destructive. Look around for local JRT clubs and see if there is a Dirt Dog or Go to Ground Competition near you. It's a great sport to get into and you don't even have to train a JRT to do it. They pick it up naturally.