They are a kind of potato onion and best suited as the storage type for the Mid-South. The thing that dropped them from favor is that splitting habit. If left to mature to full size, there's normally a dry skin between the two halves. We didn't care as they were a great cooking onion. Last I heard, the main supplier of them was somewhere in East Texas. It's one of those things which are seemingly common but can totally disappear in one year if the only grower suddenly stops.
Patnewmex mentioned that that's how shallots grow? Yep! That's why we have onions that can be grown from seed!
Martin