An aspect I've been thinking about is whether eating two devil fruits and thereby dying only has been tested on weak humans or limited testing in general.
It could indeed require that the person has a varity of certain unique features to their body in order to sustain two devil fruits.
This could be just as rare as someone having King's Haki, for example, 1 out of a million (forgot the exact number, but it's rare either way).
Perhaps it requires the King's Haki in the first place to sustain two devil fruits, and something else.
Additionally or even alternatively perhaps only certain types of devil fruits can be eaten together.
Some sort of limit could for example be:
- You can't have two same type of DF's, for example two Logia's is a no go, limiting it to eating 3 DF's in this creteria alone.
- You need a strong body (Luffy's quite flexible).
- You need King's Haki to sustain the pressure or whatever causes the death in normal people.
- It reduces ones health and thereby how long one is living. Luffy's gear techniques has alrady been warned to cause Luffy an early death or bad body.
- Requires that the person eating a second devil fruit has already a high level of control over his or her devil fruit, in order to be able to control two fruits mixed in the body at the same time, thereby avoiding daeth from loose of control of having two devil fruits in the mix.
- No one strong was ever daring enough to try it, it was only tested on weak human beings, animals or items, and never on someone strong.
- Because of the fruits sheer insane value alone few has been doing tests on two devil fruits.
If taking all these points into consideration from above, then I'm not surprised if Blackbeard would be the first ever to sustain two devil fruits, assuming they hold true.
For one being strong and crazy enough with the right kind of body and King's Haki (King's haki alone is already insanely rare), it would be rare enough for no one to discover the possibility of two devil fruits until now.
Having all this in mind, I don't think it's farfetched for Luffy to get the Mera Mera no Mi, devil fruit.
Blackbeard being the first in history could lead the way for a very few people being able to have two devil fruits, but only a very few, especially if it requires King's Haki as one of the more requirements which on its own already is super rare.
---------- Post added at 06:36 PM ---------- Previous post was at 06:29 PM ----------
Being the first person to have a second ability implanted by the only scientist capable of the procedure would be good explanation. We know Caesar has succeeded where Vegapunk failed, creating giants and artificial devil fruit. It wouldn't be a huge stretch if he also was able to succeed in giving Luffy a second ability, despite Vegapunk's past failures. It would, in many ways, be a more sufficient explanation than Teach having some sort of physical abnormality.
Not to be rude or anything, just asking, but wasn't it Vegapunk who originally created the artifical devil fruits and Ceaser did rip off copy this method?
Also it was Vegapunk who discovered the path to make giants, but he abandoned the project out of moral issues?
It's just that to me that it seems like Vegapunk is superior than Ceasar, even in Ceasers own areas of top research, which of all were shown to be continuings of where Vegapunk either stopped for some "other reason" rather than being incable of continueing.
Heck the world government might even have artificial devil fruits as well, after all, we were only told that Vegapunk did stop his reseach on giants, not that he did stop on devil fruits, as far as I remember either way.