Crayons
Iconoclast
Actually just experienced that when I ran a D&D campaign (Godsfall) with some friends a few months ago. The twins in question were the potential future goddesses of war and peace, and the remainder of our party was composed of a capitalist (war is profitable), a pillaging sky pirate, and a literal welsh corgi. Having a dog didn't exactly do much to stop the other three of us from attacking and smashing absolutely everything unprovoked. "Peace was never an option" very quickly became the motto of the game.
Cut to the final battle. Every single attempt Peace-Twin made to negotiate with the enemy has failed to that point either due to bad rolls, inability to understand, or all of us just straight up refusing to let it happen. Tired of trying to babysit all of us she slipped away with the dog, stole Sky Pirate's airship (which included a giant gold statue of himself for reasons as well as his entire crew who were apparently the only successful negotiation roll for that entire campaign - go figure), and abandoned the group. Capitalist ends up dead. War-Twin and Sky Pirate survive but are now trapped at the top of a tower with various floors of nasty automatons between them and the exit.
I love it when karma comes around in hilarious ways. All that to say, yeah, if you're going to do twins I would say it's almost required to have two separate people playing them just for the sake of planning and surprises. At least if you want to give them both the weight and impact of a main character. Trying to play both I feel just dooms you to having two incomplete halves of a single character rather than two compelling ones.
It's fine to have one person play both if they are two separate characters with two distinct personalities. It's the lumping them together into one homogenous entity that I have a problem with.