I have two similar tables in two different databases. Both tables have a column with a date and one with email addresses. Though the column names are not the same. As resul
You cannot do a UNION query across connections. You'll have to do it as two separate queries:
$emails1 = DB::connection('db1')->table('contacts_1')
->selectRaw('mail_address as email, date as created_at')->get();
$emails2 = DB::connection('db2')->table('contacts_2')
->select('email', 'created_at')->get();
$emails = array_merge($emails1, $emails2);