Another approach - just as something else to consider - is if the Backoffice needs something from the Admins - it gets it just like an API would - but in this case its an "internal" API. i've been surprised by how quickly you can wrap some data up in a JSON array and send it off with a few lines of CURL. you can even wrap database result objects in JSON and codeigniter treats them just the same as if they came from a local model. and you don't need a "rest server" - if this is a private API then you just need two lines of code in the receiving application controller method.
PHP Code:
$request = file_get_contents('php://input');
$customer = json_decode($request);
it also opens up a bunch of options in terms of staging, having different applications on different servers, etc etc.
