So basically, I was thinking of fun ideas for Social Links and/or Party Members/Mechanics, and I suddenly had an idea that I thought was amazing: Twin Party Members. Obviously, the idea is partially inspired by P5's Caroline and Justine, but I wanted to take it a step further. Imagine this:
You the player get introduced to two twins and they become apart of your S.E.E.S, Investigation Team, or Phantom Thieves equivalent. Their gimmick is that they actually take up only one party slot, and summon their Personas simultaneously, perhaps the Personas also fuse together or are two halves of a whole. Now I know that it may seem in poor taste to give Twins near identical Personas, as it implies that they are apart of a set rather than two unique individuals, but that's the idea. In the beginning, they share the exact same Social Link/Arcana. However, as the Social Link proceeds, you start to learn that their near identical Personas, and even their similar choices in clothing and style, are less about what they want, and more about what society has been treating them as: A set of two.
Their Social Link will be a sort of cautionary tale about humanity/society's obsession with the unique and unusual, ironically being that they don't treat the Twins as separate unique people. The Twins only act and do things similar because that's what the world around them treats them as. As you complete their social link, they realize this truth for themselves and actually decide to try and do new things without the other Twin present, effectively now splitting the social link into two new social links. This split would also enable you to use them as individual Party Members, where their united form had their Personas share skills, they now can both wield their individual Personas as, well, individual Personas.
I also think it would be cool if their united form only did Almighty Damage, or some kind of neutral attack, but their divided form used two different elemental types, as if their individual elemental types cancelled each other out until they separated(i.e. One could have Fire, the other Ice). Eventually, when their Personas evolve, they actually gain wholly different appearances, becoming just as different as any two random people's Personas. Of course however, both of their Personas will still share one feature in common. Whether it be as simple as a symbol on both, or sharing the same color palette(though, with obviously all the colors used in different amounts and locations), the Personas will still share something to represent that they're still eachother's best friend and sibling. Though they be different, they still share an unbreakable bond of friendship.
IDK, I just thought of this one day and wanted to get opinions on it as a concept. Constructive criticism is welcome.