Umm, pay a little more attention, fifty shades started out as a poorly written fanfic of twilight. It was then made into a poorly written regular trilogy of books that honestly I never got past the first chapter in actually reading. So glad I didn't buy it, got the fanfic version first and read two pages and delete.
Just as a lark I found all three of the buyable version and read page three and delete.
So perhaps you should try coming up with a new idea, or at least list the one you have because just saying either of those two books/series and most people above twelve cringe.
Course since you haven't actually listed an idea, I'll list one just to keep the thread from being deleted or something.
So you have the writers of the two series getting together. Picture if you will two women who have seen at least a few things. Both are married moms I'm pretty sure, but don't quote I won't look at their books. They get together to argue over a point or two and the argument gets heated. In no time both authors are tearing at each other, their clothes is being pulled off until one grabs a nipple. Suddenly they forgot about being mad and are instead incredibly turned on and fall to the floor to fuck.