I would override config.database.default in middleware

I searched for 'config.database' and 'config.database.config', but nothing found, do you mean I can create .domain1.env, .domain2.env? Then crate request over custom middleware if domain is 1 or 2?

If I'm not wrong here, you want to run single app with multiple DB? If yes, see below solution.

Stuffs

• Domain1: www.d1.com
• Domain2: www.d2.com
• DB1 for Domain1: mysql1
• DB2 for Domain2: mysql2

You can configure multiple db into your laravel specified by different domain
1) Configure database settings

//app/config/database.php
return array(
  'default' => 'mysql1',
  'connections' => array(
    # Your primary database connection
    'mysql1' => array(
        'driver'    => 'mysql',
        'host'      => 'host1',
        'database'  => 'database1',
        'username'  => 'user1',
        'password'  => 'pass1'
        'charset'   => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix'    => '',
    ),

    # Your secondary database connection
    'mysql2' => array(
        'driver'    => 'mysql',
        'host'      => 'host2',
        'database'  => 'database2',
        'username'  => 'user2',
        'password'  => 'pass2'
        'charset'   => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix'    => '',
    ),
  ),
);

2) Setup ConfigServiceProvider for use DB based on domain request. Here middlewares are called after service providers so I think this is a good way to configure database here and that's why I use service provider.

public function register() {
	//Select DB based on domain url...
	$currDomain = url();
	$currDB = 'mysql1';
	if($currDomain == 'www.d2.com') $currDB = 'mysql2';

	config([
		'database.default' => $currDB,
	]);
	DB::reconnect(); // this will help in reconnecting the database if the connection is already established. and import the DB facade. 
}

There is a more way to this but based on my knowledge this is good. Hope this help you!