RewriteEngine on
RewriteBase /
DirectoryIndex frontend/www/index.php

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_URI} cpanel/(.*)$
# Why not "-f" - because it's domain of 3d level: VirtualDocumentRoot /path/to/htdocs/%-3/ and if I use "-f" file not founds :(
RewriteCond backend/www/%1 -F
RewriteRule . backend/www/%1 [L]

RewriteCond %{REQUEST_URI} cpanel/(.*)$
RewriteCond backend/www/%1 !-F
RewriteRule . backend/www/index.php?b=%1 [L]

RewriteCond %{REQUEST_URI} !cpanel/(.*)$
RewriteCond frontend/www%{REQUEST_URI} -F
RewriteRule . frontend/www%{REQUEST_URI}

RewriteCond %{REQUEST_URI} !cpanel/(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule . frontend/www/index.php?f=%{REQUEST_URI}

.
├── .htaccess (with code I provided)
├── frontend
│   ├── www
│       ├── css
│       │   ├── base.css
│       └── index.php
├── backend
    ├── www
        ├── css
        │   ├── base.css
        └── index.php