Complex sentence: She undertook her first concert tour of the Antipodes at age 18, a months-long tour with a soprano with whom she gave joint recitals, a tour during which she met Fritz Kreisler, also in the midst of a tour.
Split into simpler sentences: She undertook her first concert tour of the Antipodes at age 18, a months-long tour with a soprano with whom she gave joint recitals. On that tour she met Fritz Kreisler, who was also in the midst of a tour.