Корневой .htaccess:
<IfModule mod_autoindex.c>
Options -Indexes
</IfModule>
<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine On
IndexIgnore */*
RewriteRule ^adminpanel/(.*)?$ /backend/web/$1 [L,PT]
RewriteRule ^([^/].*)?$ /frontend/web/$1
</IfModule>
.htaccess в frontend/web и backend/web одинаковый:
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php