I am looking for a book that I read many years ago. It is about two enemies traveling in opposite directions through time. ..one forward and one backwards. As they meets at keys points during history they both learn more about why they are enemies and who is the true villain.
You're probably thinking of Ben Bova's Orion series.
Orion (or O'Ryan and various other aliases) travels backwards through time, living multiple lives, and he battles an adversary each time. This means that his memories of why he is fighting are all in the wrong order, and Ahriman, the antagonist, knows less and less about the reasons for the conflict as the book progresses.
Now, I absolutely cannot find a good reference for this, so apologies there, but https://sfbook.com/orion.htm covers most of the basics.