I think I know this one, it is a really long series, consisted of 7 books all together. From what I can remember about this story since I read it so very long ago but it's such an amazing series that I can still remember bits and pieces of it... Buffy and Spike end up to be a part of prophecy and have a daughter which I think is Dawn. They both are the protectors of the peacemaker who is Dawn, I believe.. I really should re-read it again. It's a good 10 years since I read the first book.
It's The Peacemaker Prophecy by
Nimue Tucker :
Book 1:
Twist on Normal Again -
This is going under the assumption that in Normal Again, Buffy was jumping between dimensions. Two coexisting realities. Also, theory that every decision we make creates a different path for us, and maybe a shadow of ourselves follows that path.Book 2:
A New Life Book 3:
The Peacemaker ProphecyBook 4:
The Evil WithinBook 5:
TimeBook 6:
The OneBook 7:
Meant To BeDo you think this could be it? If it's not and you haven't read it yet... give it a go cos
Nimue wrote an amazing and absolutely wonderful Spuffy journey in this series. I know I fell in love hard reading it.

~Thia