Its not recommended that you backdate documents. When you come to an agreement that both parties agree to as at the current time and date, why would you want to backdate it?
Can you include on certain elements (if agreed of course) such as that debt created after this date xx by either party is the sole liability of that individual, but leave out reference to current joint assets (again if agreed)?
I'm not sure why you wouldn't reference current joint assets - why not set out how you'd like to deal with them and how you'd divide them if your relationship ended?