Xebec is inside of Teach's belly. Thats why Teach ate two df
BM never ate a DF; she ate a DF user, trapping both the human's soul and the devil inside her.
Similarly, BB was fed Xebec's spirit, who haunts him to the point of not letting BB sleep. BB ate the Yami Yami, but it was Xebec's remains that consumed the devil from WB's fruit.