Options +FollowSymlinks
RewriteEngine On
# deal with admin first
RewriteCond %{REQUEST_URI} ^/(admin) <------
RewriteRule ^admin/assets/(.*)$ backend/web/assets/$1 [L]
RewriteRule ^admin/css/(.*)$ backend/web/css/$1 [L]
RewriteCond %{REQUEST_URI} !^/backend/web/(assets|css)/ <------
RewriteCond %{REQUEST_URI} ^/(admin) <------
RewriteRule ^.*$ backend/web/index.php [L]
RewriteCond %{REQUEST_URI} ^/(assets|css) <------
RewriteRule ^assets/(.*)$ frontend/web/assets/$1 [L]
RewriteRule ^css/(.*)$ frontend/web/css/$1 [L]
RewriteCond %{REQUEST_URI} !^/(frontend|backend)/web/(assets|css)/ <------
RewriteCond %{REQUEST_URI} !index.php
RewriteCond %{REQUEST_FILENAME} !-f [OR]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ frontend/web/index.php 

namespace common\components;
class Request extends \yii\web\Request {
  public $web;
  public $adminUrl;
  public function getBaseUrl(){
    return str_replace($this->web, "", parent::getBaseUrl()) . $this->adminUrl;
  }
  /*
    If you don't have this function, the admin site will 404 if you leave off 
    the trailing slash.
    E.g.:
    Wouldn't work:
    site.com/admin
    Would work:
    site.com/admin/
    Using this function, both will work.
  */
  public function resolvePathInfo(){
    if($this->getUrl() === $this->adminUrl){
      return "";
    }else{
      return parent::resolvePathInfo();
    }
  }
} 

//frontend, under components array
'request'=>[
  'class' => 'common\components\Request',
  'web'=> '/frontend/web'
],
'urlManager' => [
    'enablePrettyUrl' => true,
    'showScriptName' => false,
],
// backend, under components array
'request'=>[
  'class' => 'common\components\Request',
  'web'=> '/backend/web',
  'adminUrl' => '/admin'
],
'urlManager' => [
    'enablePrettyUrl' => true,
    'showScriptName' => false,
], 

RewriteEngine On 
RewriteCond %{REQUEST_FILENAME} !-f 
RewriteCond %{REQUEST_FILENAME} !-d 
RewriteRule ^(.*)$ /index.php?/$1 [L] 

www.project.com/admin, www.project.com 

localhost/project_name/admin, localhost/project_name