What role does もう play in this sentence? > {} Does modify here, as in "already moved (lit. swam through air/space)"? So that "while X (tearing hair with one hand due to headache), the trembling other hand **already** moved"?

The here is the same as the one in "another one".

In your example sentence, [he] is scratching off his head with one arm. **The other** , shaking arm [was trying to swim in the air or whatever].

So, means "the other arm" (as opposed to "the one arm").

This can be used with arms, items, people, anything. Like

> ****
> I saw two men. One wearing a hat, the other one with a walking stick.