Sorry, you can’t do that here from what I know. But you can upload same files for both websites in their respective root folder. If you make use of a database, connect to the same database fir bith websites.
First, add “husyn.app” to your account. After you added it, open the “Parked Domains” section of the control panel and select “husyn.app” from the dropdown. Then, add the domain “husyn.co.uk”. Both domains will now use the same files.