The story begins with Wood Hite and Charley Ford, they are together until they die.
Then let's say they are reborn time after and meet again.
they are reborn in other characters until they reach the present and
they remember all past lives where they have been together. More or less that's the explanation.
The pairings in each life are:
Wood Hite/Charley Ford
Jeffrey Dahmer/James Reston Jr.
Chick Prigusivac/'Wild Bill' Wharton
Clint Barton/Justin Hammer
I hope that you like it :)