The airflow is split approximately 50% each way.

approximately 50% each way 为 is split 的“方式状语”。

50% each way 合在一起，而且有了副词 approximately 修饰，百分数之前可以省略 by。又如：

He arrived (at) about two o'clock. （有了about 可省略 at）

each way 有三层意思：1. 单程；2. 每侧；3. 每种方式