Multi-player duty support

[Enkidoh]

As I understand it, there are two reasons possibly why they don't allow this - the first is purely pragmatic. Duty Support is meant to be a safety net to allow players to complete story content at their own pace without having to be 'roadblocked' by Duty Finder not filling parties. So SE's way of thinking is if you want to do it with a friend, use Duty Finder or Party Finder and do it with other players, not NPCs. This is also the reason why Duty Support parties tend to be much slower than a normal player party run in a dungeon or trial, the system is deliberately designed to be slower so as to further encourage players to form actual parties for the content if they're more interested in getting through it as quickly as possible.

The second reason is more technical, I recall it mentioned (but I lack a citation on it) that due to how the Duty Support/Trust NPCs AI are scripted, they take the lead from the player's character, so adding in a second player confuses the AI as it doesn't know who to follow and potentially causes glitches and issues. Nothing particularly insolvable with time and resources, but again due to Duty Support being a 'last resort' for players to complete main scenario content and not the prefered means of completing content, this is something they're simply not going to look at I'm afraid.