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For whatever reason Spike was visiting Joyce and he's the one that switches bodies with Faith. Spike doesn't black out when that happens, so Faith is exposed from the start. He easily subdues Faith because she can't fight back, she has his chip.
Joyce calls Buffy.
So the story begins.
Reluctantly, Buffy acknowledges Spike's help and agrees to get his body back when Faith manages to escape. Faith runs to LA.
So, road trip to LA. Your choice if anybody else, besides Buffy and Spike, makes the trip.
Obviously, Riley has a few choices of words about it. He doesn't care Spike basically saved Joyce. Buffy gives him the boot.
Guess where Faith goes. That's right. To Angel. But she doesn't stay too long, because Buffy and Spike find her pretty quickly. At first, Angel refuses to help Spike. Buffy guilt-trips him into it. How much he's really helping, again, your choice. The search for Faith takes some time. Angel is not amused, especially, witnessing how well the two of them work toghether.
The time they spent together brings Buffy and Spike very close. They form a friendship, that obviously turns into something more. The problem is, he's still in Faith's body. There should be some awkward moments when they fight the attraction.
How long he's trapped into Faith's body is your choice, but definitely long enough for them to form a friendship.
Obviously, I want a spuffy story.

If you manage to make the two slayers become friends in the process, it'll be short of a miracle, but it's not a must.

Have fun.