The main theory is that the reason Teach can eat two df and not die is because he has more than one soul inside him. One of the souls would be Xebec soul. I said "one of the souls" because theorists believe he has 3 souls.
They also think that Teach could never make a scar in Shanks, because he was much weaker than Shanks in that time
So who make the scar in Shanks would've been Xebec