This is my understanding of how the two most visible heads of the triceps share the humerus.
The lateral head starts fully ON the humerus, and then fully leaves it.
The long head starts fully OFF the humerus, then comes to be fully ON the bone.
The bone seems to travel diagonally under the two muscle heads as they trade places. They don't share the humerus equally.
This small post is actually pretty helpful. Glad you posted it, it makes the triceps make a heck of a lot more sense now.