Unione di query da database diversi in Laravel Query Builder

Non puoi utilizzare connessioni diverse, ma puoi comunque farlo fornendo il nome del db in modo esplicito:

$q1 = DB::table('db1.contacts')
       // where(..) or anything you need here
       ->select('mail_address as email', 'date as created_at');

$q2 = DB::table('db2.contacts')
       // like above
       ->select('email', 'created_at');

$result = $q2->union($q1)->get();