Is it a good idea to split a Drupal site into multi-site one for editors/admins and another for front end users?
Is this a good idea or is it just adding an extra level of unnecessary complexity?


There is no functional advantage of creating a multi-site for the purposes of separating content administration, as that will add an unnecessary level of complexity.

Some of the issues include determining what tables to share (which you've identified). Another common scenario is that content will be rendered differently depending on the site, leading to troubleshooting nightmares. An unscrupulous developer can have multiple versions of the same module in the site code, which creates head-scratching differences.

A single site is significantly easier to manage and ensure that administrative roles and permissions are logically set given your needs.

It can be helpful to have a distinct administration theme to visually enforce the difference between a normal and administrative roles. It's possible to specify a role per theme - https://drupal.org/project/role_theme_switcher for example.