Partially additive opacity in one and the same arrow(head) in tikz

Opacity is applied to each path separately and hence overlapping paths look "odd". For this reason, paths need to be connected when the same opacity needs to be applied to all of them. To achieve this, use a scope with the option [transparency group]. This key applies the proper opacity with no overlapping. In your case,

\begin{scope}[transparency group, opacity = 0.5]
\node (a) at (0,0) [] {};
\node (b) at (100pt,0) [] {};
\draw[->,line width = 20pt] (a)--(b);
\end{scope}

In the pgf manual, this is explained in section 23.5, Transparency Groups.

As for your question regarding several opacities: Opacities are generally multiplicative. You can set the way of multiplication (regarding colour, cut-off and son) via [blend mode =], as detailed in section 23.3, same manual. (Again, the key has to be applied to a surrounding scope, not the path itself.) Manual can be found here: http://ctan.org/pkg/pgf

Use transparency group

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
  \thispagestyle{empty}
  \begin{tikzpicture}
    \begin{scope}[transparency group, opacity=0.5]
      \node (a) at (0,0) [] {};
      \node (b) at (100pt,0) [] {};
      \draw[->,line width = 20pt] (a)--(b);
    \end{scope}
  \end{tikzpicture}
\end{document}