Two Heavens One Style is a school named for its founder, Miyamoto Musashi. In Japanese it is the Niten school, as one says the Yagyū school. 一流 is one word, a school, and 二天 is Musashi himself.
The English title reads the four characters one at a time. It is a good title once you know that Niten is a name.
The English is David K. Groff's, from his The Five Rings of 2012. 二天一流 literally means "Two Heavens One Style" and is composed of the kanji 二 (read ni) meaning "two", 天 (read ten) meaning "heaven, sky", 一 (read ichi) meaning "one", and 流 (read ryū) meaning "style".
Musashi called his school 円明流, Enmei, in his youth and 二刀, two swords, in his last years; 二天, two heavens, was his own Buddhist name, Niten Dōraku. The school's name 二天一流 is first written in the copies of his five scrolls, made after his death, and the dictionaries date the renaming to about 1666.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 二天一流.