2-That devil fruit is a Cerberus Mythical Zoan.
3-That devil fruit allows the user to “fuse” in some way or “add” up to two other people to oneself (say like Cell did with A-17 and A-18 in Dragon Ball Z).
4-Thatch ate the Yami Yami no Mi.
5-Teach incorporated Thatch into himself after the latter ate the Yami Yami no Mi.
6-Teach incorporated Whitebeard into himself after killing him (Or while he was still agonizing in his last moments almost dead to be more precise).
7-Incorporating the “essence” (soul, spirit, life force, X, whatever) of an ability user that by this process ceases to exist as an alive individual “captures” the powers of the devil the same way eating a devil fruit does.
8-As a consequence, Teach acquired the powers of the Yami Yami no Mi through incorporating Thatch and the powers of the Gura Gura no Mi incorporating Whitebeard.
9-Teach could possess additional devil powers BECAUSE the holder was still another person and not just one person possessing multiple powers.
1-Teach would possess the powers of 3 devil fruits. One of each main class.
2-Teach would be multiple people. Specifically, 3 people currently, 2 people at the time Luffy and Zoro noticed.
3-Teach is one of the characters that has changed physically the most over time. And every time he’s prooved to have a new devil power his appearance has become more similar to that of real life Blackbeard.
4-The 3 characters that are named after real life Blackbeard would be one and the same, as it makes no sense for 3 different Blackbeards to exist.
*Theory by The_Real_Katakuri