You can have multiple controllers in your module. So make a folder admin with admin controller in your module's controller folder.
mymodule
- controller
- home.php
- admin/admin_dashboard.php
- model
- model.php
- view
- home_view.php
- admin/admin_view.php
To access the modules
echo Modules::run('mymodule/home/methodname');
echo Modules::run('mymodule/admin/admin_dashboard/methodname');