“The question, which comes before us upon a certificate of a division of opinion of the judges of the Circuit Court of Massachusetts, is this, whether two or more persons, jointly charged in the same indictment with a capital offence, have a right, by the laws of the country, to be tried severally, separately, and apart, the counsel for the United States objecting thereto, or whether it is a matter to be allowed in the discretion of the Court.