We have a case of "lost in translation" here. 即尺, basically a quickie fellatio, is translated as "double fellatio."

The translator probably did this due to the context, in which Rin's confused due to unawareness of sex terms. Having the guy speak in layman's terms breaks the context, and 即尺 has no equivalent in Korean. Which only leaves swapping 即尺 with another term to retain the context.

Transliterating 即尺 might have worked, but that requires a translator's note, which inelegantly leaves clutter on the page. Kudos to the translator here; other people overuse notes, which should be used sparingly.

Base +5