'connections' => [

     'mysql'      => [
         'driver'     => 'mysql',
         'host' => env('DB_HOST', '127.0.0.1'),
         'port' => env('DB_PORT', '3306'),
         'database' => env('DB_DATABASE', 'forge'),
         'username' => env('DB_USERNAME', 'forge'),
         'password' => env('DB_PASSWORD', ''),
         'unix_socket' => env('DB_SOCKET', ''),
         'charset' => 'utf8mb4',
         'collation' => 'utf8mb4_unicode_ci',
         'prefix' => '',
         'strict' => true,
         'engine' => null,
     ],

    'yoursql'  => [
         'driver'     => 'mysql',
         'host' => env('YOURSQL_HOST', '127.0.0.1'),
         'port' => env('YOURSQL_PORT', '3306'),
         'database' => env('YOURSQL_DATABASE', 'forge'),
         'username' => env('YOURSQL_USERNAME', 'forge'),
         'password' => env('YOURSQL_PASSWORD', ''),
         'unix_socket' => env('YOURSQL_SOCKET', ''),
         'charset' => 'utf8mb4',
         'collation' => 'utf8mb4_unicode_ci',
         'prefix' => '',
         'strict' => true,
         'engine' => null,
     ],
],

-mysql
-yoursql

DB_CONNECTION=mysql
DB_HOST=127.0.0.1
DB_PORT=3306
DB_DATABASE=base_de_datos_principal
DB_USERNAME=usuario_base_datos_principal
DB_PASSWORD=pass_usuario_base_datos_principal


YOURSQL_CONNECTION=mysql
YOURSQL_HOST=127.0.0.1
YOURSQL_PORT=3306
YOURSQL_DATABASE=base_de_datos_secundaria
YOURSQL_USERNAME=usuario_base_datos_secundaria
YOURSQL_PASSWORD=pass_usuario_base_datos_secundaria

<?php namespace App;

use IlluminateDatabaseEloquentModel;

class TUMODELO extends Model {

    // Conexion que va a usar este modelo
    protected $connection = 'yoursql';

    ...

}