I think the plan was that once the new platform 4 was constructed as an island platform, only one platform face was required for the use of the heavy-rail train services, leaving the other platform face for the eventual use of the Manchester Metrolink system.
Whatever the planning was, and whatever the map jfollows posted shows, I'm pretty confident that the reality on the ground is that there are 6 platform faces, platoforms 1-4 on the heavy rail and two platforms (which don't appear to display numbers, lets call them 5 and 6) connected to the manchester metrolink.
I was just wondering about how they were used operationally, particularly in light of the ticket barriers installed on the heavy rail side in 2018, which appear to include platfor "5" inside the gated area. It sounds like the answer is they avoid using platform "5".
I’d expect that’s the reason for it
I'd have more expected the reason is that a single platform terminus on a double track line is an operational risk.
Bury, Rochdale, Ashton, East didsbury, Altrincham and the Trafford center also all have two metrolink platforms. Eccles seems to be the only metrolink terminus that doesn't.