What information should we include in our letters. Should our co sponsor write something also? He is in the uk now and I'm in the us. Do we both need to write separate letters?
You mean letter of introduction by you and letter of spponsorship by your UK fiancé(e)? Just outline your relationship from your point of view from first contact, first meeting and subsequent meetings, deepening relationship and engagement, to your wedding plans and hope for the future - to settle permanently in UK. Just jot down salient facts, such as dates and duration, and a growing sense of commitment and shared responsibility (like opening a savings account for your future life together).
You mention co-sponsor. That means someone in UK other than your partner who has agreed to offer you support, such as free accommodation or regular monetary gifts. Then they should write a letter formally offering you support, with supporting documents such as mortgage and bank statement.