The ``double vowel'' paradigm [P. F. Assmann and Q. Summerfield, J. Acoust. Soc. Am. 88, 680--697 (1990)], in which recognition for two concurrently-sounding vowels is measured over various fundamental frequency (F0) separations, is applied to natural musical instrument sounds. The findings of the previous work, in which recognition reaches peak by 1 to 2 semitone difference, is replicated here, with about 35% greater accuracy overall for musically experienced over naive listeners. Attention is given to special cases of instruments that are frequently confused for one another even when F0 separations are wide (e.g., clarinet and English horn, or flute and French horn), since such combinations may have special value in musical orchestration. In another condition, concurrently sounding instruments are adjusted in level by listeners so the two sound equally salient. The double-timbre study is then run with variable relative levels between the two instruments. Translations between the effects of semitones separation and difference in level are explored.