The basic gist of it was (From what I can remember) is that in Suikoden, you have the "True Runes" and their off shoot counterparts. From what I remember, each true runes can bare two other runes (maybe three), and when those runes come together they form a "True Rune".
So the theory was that is how Blackbeard ate two devil fruits, because those two fruits came from on singular fruit. So one person could technically eat two fruits as long as the came from the Original fruit.
Then I went to to point out how there is multiple devil fruits in One Piece that are off shoots of each other, or rather "Superior/Inferior" fruits.
Mr. 5 and Gladius' fruits.
Miss Valentine, and Machvises fruits.
Ace and Akainu
Aokiji and Monet
There was a few others. Oda mentioned them in an SBS, which is what prompted me to make the theory, cause I just so happened to be replaying Suikoden at the time that the SBS dropped, and I was like, wait a minute....