Knack apps are independent and cannot be linked in the builder. They operate in isolation without being connectable to share data between them.
You could achieve what you are looking for by using Integromat to bridge between both apps via Knacks API. It could look for a new account to be added and then setup the same user in another app. It could also update if a user leaves to remove access in the other app.
Providing the specific solution here is not possible and if you’ve not used Integromat then there is a learning curve to go through. It would be a fairly easy fix as along as the person is using the same email or has a unique identifier in both apps so they can be looked up.
You may also consider building out in one app so you have all the tables in one place and then have views based on user roles. I have a few client apps that have many sections (HR, Operations, Finance, Compliance, Vehicle Management etc) and no one person see’s every section as it’s not relevant to their job role.