Tanjiro makes history by combining two breathing techniques in Demon Slayer 

In Demon Slayer's Entertainment District arc, Tanjiro, Inosuke, Zenitsu, and the Sound Hashira, Tengen Uzui, finally got to face the Upper-Rank Six demon Gyutaro. A demon who is terrifyingly overpowered, capable of handling two demon slayers at once.

In the latest episode, "Defeating an Upper-Rank Demon," Tanjiro displayed his overwhelming resilience, performing a never-seen-before technique that raised the bar of excitement for viewers.

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How does Tanjiro combine two breathing techniques mid-battle in Demon Slayer?

Tanjiro was completely exhausted after using the Sun Breathing in a row against Daki. Instead of surmising the fact that he couldn’t keep up with the fight, he strenuously forced himself to be useful.

As Tengen was unable to help Hinatsuru from the clutches of Gyutaro, Tanjiro took charge by combining his Water Breathing technique with Sun Breathing as a last resort which resulted in rescuing Hinatsuru from his confinement in conjunction with severing the arms of the demon in a blink.

The idea behind combining the two techniques was that Water Breathing seemed weak against Gyutaro but is agile. On the other hand, Sun Breathing was immensely strong but required much strength and stamina from the user.

By combining these two techniques, he was able to move fast and land an effective blow before the Gyutaro could react.

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Is Tanjiro the only demon slayer who could combine two techniques?

By far, Tanjiro is the only person who has performed this technique and it was certainly a powerful combination. It might be possible for a demon slayer to merge two breathing techniques or maybe more than two.

But the crux is that a user must be able to perform Sun Breathing because it is a pre-eminent technique, out of which others have branched out.

Tanjiro is a beginner at using Sun Breathing, as it drains most of the energy and strength. Moreover, extreme usage of this technique could restrain the movements of the user for a short period.

The progenitor of Sun Breathing, and also the legendary demon slayer, Yoriichi Tsugikuni, was only able to alter his technique to suit the abilities of others by adapting them to their strength. As there is no mention of him combining two techniques so far, the argument can't be concluded.