All I can remember is that in this fiction, spike is older than angel because he was turned by darla first. Than the gypsy curse worked for spike as well as angel.

I also believe there is a sequal in the making but still a wip. Any ideas?

JMjunkie
I believe you're looking for Si Primo Venit by KittenOfDoomage. (I have no idea how to post links.) It's complete, and she already has a sequel of it out, though it's not that far in yet. There was also another story not by her but by another author over at EF who responded to the "Spike older than Angel" challenge as well, but I don't remember their name.