Complex sentence: Accrual basis is a method of accounting ("accrual accounting") whereby economic activities (rather than cash flow) of financial events are considered, because of two complementary principles, which (together) determine the point, at which expenses and revenues are recognized.
Split into simpler sentences: Accrual basis is a method of accounting ("accrual accounting") whereby economic activities (rather than cash flow) of financial events are considered on two complementary principles. They both determine the point, at which expenses and revenues are recognized.