As a rule an engine with 2 rings are more inline with an industrial application like the Zens are and are prone to last a very long time. Granted the extra ring has some extra drag and can cost a little performance. The idea of using just one ring in the top groove is fine, but it makes me wonder how much crap and crud will get trapped in the lower ring groove over time and maybe create it's own set of issue's down the road. Myself depending on how much use the engine has on it now would dictate weather I replaced just the top ring or both.