A simple idea that explains everything weird about Blackbeard:

The Davy Clan (Teach’s lineage) are born with two hearts, and Devil Fruits bind to the heart.

If Devil Fruits attach to the user’s heart/soul, then a person with two hearts can anchor two Devil Fruit souls.
This fits perfectly with canon.

Why this works with official lore

1. Blackbeard’s “abnormal body”

Canon repeatedly says Teach’s body is “strange” and “not normal.”
Two hearts = unique physiology.

2. Devil Fruits bind to the heart

Vegapunk, Law, Big Mom, and Brook all confirm DF powers are tied to the soul, which is anchored in the heart.

One heart = one fruit.
Two hearts = two fruits.

3. Explains why he never sleeps

Two hearts → two circadian rhythms → one consciousness always awake.

4. Explains his extreme pain

Yami Yami no Mi amplifies damage.
Two hearts = double pain processing.

5. Explains the Davy Family persecution

The Davy Clan was hunted for 800 years.
A race capable of wielding multiple Devil Fruits would terrify the World Government.

6. Explains the three‑skull Jolly Roger

Three skulls =

• Heart 1
• Heart 2
• The stolen Gura Gura soul

7. Explains Rocks D. Xebec

Rocks was born “Davy D. Xebec.”
If the clan had two hearts, Rocks had it too — explaining his monstrous strength.