Possible Clues:

• Two of the same kind, archaically
• American author, Mark 2
• Mark the Finnish author?
• Mark as adult visiting his very closest relative
• Tom Sawyer author, d. 1910
• Two fathom pseudonym
• The two at opposite ends that will never meet famous author?
• Who said 'Familiarity breeds contempt - and children'
• Old pair
• "The Prince and the Pauper" author Mark