Did the orphanage not tell them they were twins? Or that his name wasn't Hitori? Or did they just take "Hitori" right on the spot and didn't even tell the people running the place. Well. This old couple isn't the smartest anyways, so they probably didn't question it too much.
I wonder, when Migi and Dali finish their quest, will they adopt both if them? Like "hey we found out you have a twin. You are such a good kid, I bet your twin is too"
Cue them forgetting they're no longer both Hitori, and swapping roles all the time