would it be possible to have duty support for small parties instead of only solo?

[Turnintino]

Because the party leader might not always be the recipient of certain mechanics.

Like stack markers that always specifically target the player in certain trust dungeons. Depending on how the companions are coded, they might just be hard-wired to auto run to the player when the stack marker based mechanic goes out, not necessarily because they know you have the stack marker specifically. Or if a mechanic involves a spread out aoes, they may be hard coded to check the sole player's position, which wouldn't translate to if there were multiple players, where they might just run on top of another player who isn't the leader.

Not saying its impossible to get the trusts working with multiple true players mind you. Just that depending on the AI logic already implemented, it may require effectively re-writing their mechanic based AI script for an entire dungeon's boss mechanics. Considering they only get 6 or so dungeons updated every patch currently with just single player support, for them its more than likely not worth the effort to go back and potentially re-do every dungeon again to support what is arguably a very niche feature when there's plenty more things on their plate, like looming expansion development.

This doesn't seem to be the way the AI functions in at least some cases, though. Where predetermined AoE markers, or any similar positional mechanics, are concerned, they have choreography programmed into their behavior that the player is forced to work around. Under most circumstances, they won't even attempt to adapt if, for example, your markers overlap in a way that could get you both killed. And when you get stackers, (at least in the duties I've done with them) they will conglomerate in a similarly predetermined location in the arena, regardless of where the player is -- even if you're the one with the marker.

I can't remember the name of the dungeon, I'm sorry, but the one where each party member has to get inside one of the bubbles to avoid the arena-wide AoE? The only time I've noticed the AI attempt to adapt itself around the player in an obvious way was when I kept stealing the bubble G'raha was programmed to pick first lol (sorry, Raha, I love you). But of course, for whatever reason, they're programmed to cut that mechanic pretty close, so he was always too slow to make it to another bubble in time.

Point being, they seem to pay less attention to the player than one might think, so I don't think introducing another into the mix would deeply impact that aspect of their programming, at least.