I don't want to set it up this way, as keeping track of who posted when can be difficult for us moderators. What I can do is we go four rounds; Sarah's post will go up first; and you will be free to follow with your opening post, as Sarah says, as long as you need. The next three rounds will be alternating with a 500 word limit; Sarah's last post will be the ninth and your the tenth; however I will keep these moderated until you both have posted, then I will approve both at the same time. as with the opening posts there will be no size limit on these closing posts.
Is that acceptable to you both?