Chimera!
It was an extremely rare medical condition in which one individual developed from two or more embryos formed from different fertilized eggs!
People with this condition could have multiple immune systems, blood-flow patterns, and even distinct DNA, depending on how many extra embryos they had!
As unbelievable as it sounded, the condition really existed. It was just incredibly rare!
Even on Earth, with its booming population, the Scientific Community estimated that there were only around a hundred cases worldwide!
The One Piece World had an even smaller population, so cases were rarer still. But Teach happened to be one of them!
Only after studying him closely did Loa realize that the left and right sides of Teach's body had come from two different embryos! Their skin was the same color, and years of training had long since blended their auras beyond an ordinary person's ability to distinguish them. But at this distance, Enemy of All Life told Loa unmistakably that the two sides were entirely different!
The left and right halves of Teach's body were, in truth, two different people!
The only part that had fused completely was Teach's brain, which housed the consciousness ruling the body! That consciousness belonged to the left half!
Everything clicked for Loa!
No wonder Marco had called Teach's body a freak in the original story. He looked like one person, yet gave off two distinct auras. Others might not notice, but he could hardly hide it from crewmates who knew him inside out!
In other words, the Dark-Dark Fruit Blackbeard had eaten inhabited only one half of his body! His other half could accept the power of another Devil Fruit!
Combined with the Dark-Dark Fruit's power to draw out someone else's fruit, that was why he had plotted to steal Whitebeard's Tremor-Tremor Fruit