'connection' => [ ... ],
'connection2' => [ ... ],

'ConnectionInterface' => [
  // When namespace starts with `App\Secondary`, pass `connection2` for `ConnectionInterface`.
  [
      'namespace' => 'App\Secondary',
      'definition' => Reference::to('connection2'),
  ],
  // When parameter name is `secondaryConnection`, pass `connection2` for `ConnectionInterface`.
  [
      'parameterName' => 'secondaryConnection',
      'definition' => Reference::to('connection2'),
  ],
],