Possible Clues:

• Known only to the two of us
• Confidentially, our inheritance may be so divided
• Secret-teller's intro, and an apt title for this puzzle
• All that divides us is said in private
• Don't tell, but the gatepost's gone missing!
• Involving the first two people in confidence
• ''Let this be our little secret''
• An expression of mutual confidence
• Confidentially, she is in the dictionary, but he isn't
• What our secret should be