Make sure you check with your county /city zoning desk to learn if there are any new requirements that spell out when shared septic systems are no longer permitted, or eventual failure of the current septic syste and think about spelling out maint of the shared drive , and under what situations if any the use of the the driveway would expire? ( like township growth that creates a new road , sale of property, one party creating a home business that increases the use of the shared item to a point where the overuse creates unfair cost to other party, etc ) The services of a atty for this would be the best way to go so you dont end up with agreeements that are vague or so poorly written that it creates larger legal cost down the road.