So this is something I can't wait for oda to answer what went down Btwn these too that caused shanks to get his scar.
I have 2 speculations on this
1. That shanks inherited the book of all devil fruits from his days as a roger pirate and Blackbeard needed to obtain this book as to get the right devil fruit for his goals. So there was a tussle at some point that resulted in the scar. My support can go to the beginning of the series when shanks told Luffy immediately upon eating the devil fruit exactly what it was indicating his knowledge of the fruits as opposed to someone like spandam not knowing what the two devil fruits he gave cp9 did.
2. Is Blackbeard heard that shanks had a devil fruit that matched the description of the yami yami no mi but upon tracking it down found out it was the Gomu gomu no mi instead and shanks got the scar protecting his prized possession while bb slinked away into the night