upon one another = on one another = on each other

They rely (up)on one another. 他们相互依赖。

Those people look down (up)on one another. 那些人互相瞧不起。

一个普通的介词短语。在“相互代词” 前用了 upon，根据需要也可以用其他介词。例如：

We learn from one another (from each other).我们相互学习。