Could this please be an option for Arcane? This was probably the most fun I had with Summoner which was essentially me having a twin. I felt like the ice climber brothers from Smash bros.
Im confident there will be ways to do exactly this with many backgrounds.
Its your inner conscience, given form by the divine (Angel). Either that, or a manifestation of your base desires (Demon).
Its a Simulacrum, made in your exact image (Construct) .
Its a Doppelganger from outer space, or some sort of pod person (Abberation).
Its the result of a fey prankster, who took a liking to you and started imitating you as a joke. Now its just a thing (Fey).
Theres not really a need for the Twinned archetype mechanics anymore (the item rules for Eidolons are super simplified), and there's a lot of cool hooks for pulling off and explaining the gimmick.
I highly encourage you to do this and come up with a cool explanation- but I sincerely bet that explanation will fit one of the other base types.