RILM abstract:" Original source material on historical tongue-strokes describes "paired syllables", or double-tonguing, as contrasted to single-tonguing. Patterns place the accent on the first, second, or both parts of the pair. Double-tonguing uses a rebound motion to enhance the velocity of Articulation; it reinforces appropriate metrical patterns, but is more often used to achieve speed in extended (and therefore fatiguing) runs of quick notes. Descriptions of double-tonguings began in the early 16th c. and continued to ca. 1827, and were used in Italy, Germany, Holland, France, and England."
"Tu ru or not Tu ru: Paired Syllables and Unequal Tonguing Patterns on Woodwinds in the Seventeenth and Eighteenth Centuries,"
Performance Practice Review:
1, Article 5.
Available at: http://scholarship.claremont.edu/ppr/vol10/iss1/5