Final answer:
Austrian composer Arnold Schoenberg was known for alternating music between two groups and pioneering atonality and the twelve-tone scale, challenging traditional Western music expectations.
Step-by-step explanation:
The composer known for alternating music between two groups of voices or instruments and pushing the boundaries of music was the Austrian composer Arnold Schoenberg. Schoenberg's approach to composition was marked by the use of atonality and the development of the twelve-tone scale.
His work was a significant departure from the traditional scales used in Western music since the Renaissance and often included dissonance and unexpected notes, challenging listeners' expectations.