Fantastische Hure said:
They perfected it even further, thus the boost increased. One of the manga scans I read said "they live to outdo each other, that helps too" or something like that. Basically it means they push each other to become higher and when you have someone pushing you to become better you become better and that means the fusion will also benefit from you becoming better since you are one of its components.
That's fine then, I was only justifying the varying boost for Metamorian's fusion on people's list. Btw, as a nitpick, Piccolo only stated the steps went down perfetly, meaning they didn't commit any coordination error like they had before, leading to those failure results. It doesn't necessarily mean they took the steps to a new level.
As for the "living to outdo each other", that was only VIZ. This is the original dialogue:
Chapter: 504 (DBZ 310), P9.2-3
Context: as Vegetto beats up on Gohan-absorbed Boo
Kaioshin-Kibito: “H-he’s strong!!! Majin Boo there is helpless!!! To think that merging with the Potara would be this incredible…!!”
Elder Kaioshin: “Idiot, it’s because it was those two that they were able to go so far. Two of the top 3 masters in both the living world and afterlife have merged, after all. What’s more, two rivals have joined together. That’s definitely strongest.”
Notice that Elder Kaioshin specifically highlighted Goku and Vegeta's powers in the first part of the comment. The second part (the one which refers to the rivalry) is introducced with a "what's more", meaning it's separated from the first one. If rivalry were to be taken in the context of "living to outdo each other", wouldn't he just be reiterating the main reason highlighted on his comment -- their strength? That's why I think it's to be taken more in the context of "if two competitors/rivals perform this, the result will be stronger than if performed by ppl with the same strength but that don't share a rivalry".