How can I use sub-controller WITHOUT providing the directory name?

I've read the docs and the forums, but I haven't yet figure out how to do this:
http://siteA.com/ControllerA/someMethod --> handled by Controllers/ControllerA

http://siteB.com/ControllerB/someMethod --> handled by Controllers/SubDirectory/ControllerB

Controllers
--ControllerA.php
--ControllerA1.php
--SubDirectory
----ControllerB.php
----ControllerB1.php

The typical solution is to access: http://siteA.com/SubDirectory/ControllerB/someMethod
But I don't want to specify SubDirectory, esp. since the controller is to be accessed via a different domain.

Note that this worked in CI 2.X

Thanks!