You're being too literal. You're hearing "when two Teigu users fight, one of them dies" and assuming that this is an infallible statement of an inviolable rule of the universe. That's silly. If you consider the actual context of when this statement is made, or just be less literal about it, you'd realize the statement is merely saying "these things can be pretty lethal when used to that end, and similarly for the people who possess them". In even shorter terms: "they're powerful, dude!"
Consider some similar examples:
"Two men enter, only one man leaves!" ... and then the other one leaves a little later and less happy.
"There can be only one." ... unless we simply resolve to leave each other alone. Also, are we aliens, or what, because there's probably lots of us if so.
"One ring to rule them all." ... but utterly fail to have any usable power over the majority of those it was supposed to rule.
"The greatest sword ever forged." ... because we have totally measured all swords across all history and agreed upon an uncontroversial and objective measurement of greatness for said swords. Also we clearly defined what a sword is ahead of time, so this is not an ambiguous statement.
"You pay for the whole seat, but you'll only need the edge!"
None of these things are literally true and inviolable and any violation of them in a story is not a plot hole. Indeed, their violation in stories is usually the entire point. It demonstrates how the characters are not infallible, and the heroes and top-tier villains need to demonstrate how amazing they are by overcoming them. They are either statements of potent intent (Sauron wanted the ring to rule them all, but Dwarves didn't care and Elves were too savvy and resistant), or statements to underscore the gravity of the situation (the Thunderdome is serious business, so best get amped).
Now, constant violation of the statement can take the edge off of something. If you routinely find yourself needing far more than the edge of your seat, you will become inured to statements to the contrary. You may have expected Teigu fights to be routinely deadly and brutal, but if you see several things to the contrary you will start taking the matter less seriously. This would be a flaw in story structure (or the reader/viewer just being jaded), but not a plot hole.