Both the book series "American Mercenary" and "Shadow Strike" by Jason Kasper have David Rivers as main character. How're they linked?

What I mean is, is one a prequel of the other? Or both in parallel universes? Whatever, I can’t seem to explain why the main character should appear in two separate series. I even looked up at the plots, and they look similar. In both cases the MC is involved in a killing program, one by the CIA, the other by a global crime syndicate.