Complex sentence: Biographer Susan Curtis speculated that his mother's support of Joplin's musical education was an important causal factor in this separation; his father argued that it took the boy away from practical employment which would have supplemented the family income.
Split into simpler sentences: Biographer Susan Curtis speculated that his mother's support of Joplin's musical education was an important causal factor in this separation. His father argued that it took the boy away from practical employment that would supplement the family income.