My 2 cents: to be a joint performer, the combined performer name must contain full aliases of both underlying artists. In this case Kenny Loggins never performed as "Loggins", so the condition is not fullfilled, so it must be an separate artist.
The main reason joint performers were introduced was to avoid creating too many combination artists.
Seems useful as some rule of thumb but certainly not determinative. Scroll down the entries for Hall & Oates and tell me what you see regarding performing artist credits.
This isn't some rare instance. If the current guideline is clear and effective, why is a Manager and a very experienced Editor having a dispute over a related question? The purpose of my original post wasn't just about L&M, but the more general question that would apply to all artists.
If there is a grey area left for editor discretion, why would an error report be filed by someone who should know this already?