If you are developing a site using a modular (component) approach, and if you allow the likelihood that components will intersect between projects, then admin assets should be stored in the same place as the main site. For example, if you are developing an online store, then you may have many components that will intersect between the admin area and the main site. For example, product cards. On the main site, these cards are displayed in the product catalog, and in the admin panel - as a preview. Such an approach will allow you to get rid of the duplicate code and arrange the admin panel in a style similar to the main site. In addition to serious components, you can also protect yourself from repeating code when creating such trivial things as shapes, buttons, icons, etc.
If you do not use modularity, that is, if your components are dependent on each other, then divide into two projects.