Should we use single structure or multi structure for backend and frontend

I want to develope backend and frontend separate, and wonder that should I use single structure for each frontend and backend , or I should use multi structure for BA , FO ?

Please give me your opinion . Thanks

For something I'm working on, I'm using 2 modules, backend and frontend, except the model files and other program logic are in a shared library folder outside both modules. One thing about multi module is that you can customize each module, like routing. View templates are in different directories too, so the admin site can look different and be developed indepedently. For example, in the initial push to completion, the backend can look bare, where priority is placed on the frontend.

Initially I thought maybe some view partials like paging is the same and have set it up to be so, but I find that they may need to be different, the admin site may need more advanced controls, while the front end needs to be a lot simpler. I've not work this out yet.