you can change the the default env variable names, it will not break the functionality of you appication

you can change your database.php to something like this:

'connections' => [
    'mysql' => [
        'driver' => 'mysql',
        'host' => env('MYSQL_DB_HOST', 'localhost'),
        'port' => env('MYSQL_DB_PORT', '3306'),
        'database' => env('MYSQL_DB_DATABASE', 'iranad'),
        'username' => env('MYSQL_DB_USERNAME', 'root'),
        'password' => env('MYSQL_DB_PASSWORD', 'mysql'),
        'charset' => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix' => '',
        'strict' => true,
        'engine' => null,
    ],

    'mongodb' => [
        'driver' => 'mongodb',
        'host' => env('MONGO_DB_HOST', 'localhost'),
        'port' => env('MONGO_DB_PORT', '27017'),
        'database' => env('MONGO_DB_DATABASE'),
        'username' => env('MONGO_DB_USERNAME'),
        'password' => env('MONGO_DB_PASSWORD'),
        'options' => [
            'database' => 'admin'
        ]
    ],

],

then on the .env define each variable name

#sql 
MYSQL_DB_HOST=...
MYSQL_DB_PORT=...
MYSQL_DB_DATABASE=...
MYSQL_DB_USERNAME=...
MYSQL_DB_PASSWORD=...

# mongo
MONGO_DB_HOST=...
MONGO_DB_PORT=...
MONGO_DB_DATABASE=...
MONGO_DB_USERNAME=...
MONGO_DB_PASSWORD=...

on your models define protected connection attribute: this should be either of your connection names you defined on the database.php

    protected $connection = 'mongodb';

the only problem for you here would be implementing relations between databases, which is impossible, you will need to write your own queries for that.