It is possible that yami DF was elbaph's legendary's DF?
TAC made a similar comment as well ^^
Narratively given Teach is being built up as the final antagonist on the pirate side, it would make for a nice constrast, if we assume that the other one is the Nika Fruit. However, I'm not sure how useful this route would be to the overarching objective of toppling the WG. Teach and Luffy are never going to work together to take down Imu. They are not simply rivals but arch-nemesis in the same vein as Rocks/Roger or Imu/Joyboy so the two working in tandem is hard to believe to say the least but I guess Oda could write it in the form of a battle royal of sorts in which Luffy and Teach are not necessarily helping each other but choose to attack the WG at the same time for different reasons.
Still there's the question of how the legendary DF on Elbaf found it's way to Tatch as it'd imply that the pervious wielder died.
Not really a fan of the Yami Yami being it to be honest. Not to mention that the Yami Yami doesn't really have any ties to Elbaf lore so one would have to question why it was even on Elbaf in the first place.