config/database.php

'mysql' => [
    'read' => [
        'host' => 'readdb.example.net',
    ],
    'write' => [
        'host' => 'writedb.example.net'
    ],
    'driver'    => 'mysql',
    'database'  => 'database',
    'username'  => 'root',
    'password'  => '',
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
],

read, writeを指定してあげればOK