Onions will divide after their first year. When they flower they'll also form a new bulb on the side of the old bulb that dies after flowering. I've mainly seen them divide in two, but different types will behave differently, and damage to the bulb or some kind of growth deformity may also result in a plant that grows in unusual ways.

Walla Walla's normaly have more than one center and each center will seen up a sprout.more sprouts=more seed. YOPPER