You use two vertical "pipes" for or.

||

http://www.w3schools.com/JS/js_comparisons.asp

It's nice, because it "short circuits". That is, if the first statement is true, it won't bother to evaluate any more statements. So if you have a statement that is likely to be true most of the time, more so than another statement, put that first for more efficient code.

What you currently have is:

if (Input.GetAxis("P1_HorizontalAim")) || (Input.GetAxis("P1_VerticalAim")) {
    isTurning = true;
}

First, the entire conditional part of the if state needs to be in parentheses. Second, I suspect that Javascript wants a boolean value in the if - and Input is returning a float. Try this instead:

if ((Mathf.Abs(Input.GetAxis("P1_HorizontalAim")) > 0.1) || 
    (Mathf.Abs(Input.GetAxis("P1_VerticalAim")) > 0.1)) {
    isTurning = true;
}

Update: confirmed that, yet again, Eric5h5 is right - this code does work, I tested it:

function Update () {
    if (Input.GetAxis("Horizontal") || 
        Input.GetAxis("Vertical")) {
        isTurning = true;
    }
}

The only question I would have, is if physical joysticks can generate spurious values, or whether Unity filters them out below a certain threshold.