One of the main ways an issue with the first stage presents itself is with a failed second stage. Therefore, a leaky, or otherwise failed, second stage might actually be a first-stage issue. Unfortunately, the two second-stages on a regulator are usually set at a different cracking effort, or function differently. So, one second-stage working and one second-stage failing doesn’t allow for an easy diagnosis.
To allow for the best diagnosis of an issue, we want to see the regulator system as a whole.