Double-headed arrows

See the Section 74 (also 23) of PGF documentation (mine is version 2.10) to obtain a lot of arrow tips.

Just put on preamble

\usetikzlibrary{arrows}
\pgfarrowsdeclarecombine{twotriang}{twotriang}%
{triangle 90}{triangle 90}{triangle 90}{triangle 90}

and then you can use \draw[twotriang-twotriang] (0,0) -- (2,0);

Here is the output.

Here is a part of the documentation

A possibility is to use postactions and shorten

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}

\begin{tikzpicture}[font=\sffamily\small]
\draw[-triangle 90, postaction={draw, shorten >=4pt, -triangle 90}] (0,0) -- (1,0);
\draw[-triangle 90, postaction={draw, shorten >=-4pt, -triangle 90}] (0,-.5) -- (1,-.5);
\end{tikzpicture}

\end{document}

In the first case the second head is added before the original one, in the second is added after. This is controlled by the positive or negative shorten value.

If you want to typle less you can do:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}

\tikzset{dblb/.style={postaction={draw,shorten >=4pt, -triangle 90}}}
\tikzset{dbla/.style={postaction={draw,shorten >=-4pt, -triangle 90}}}
\begin{tikzpicture}[font=\sffamily\small] 
\draw[-triangle 90,dblb] (0,0) -- (1,0);
\draw[-triangle 90,dbla] (0,-.5) -- (1,-.5);
\end{tikzpicture}

\end{document}

same result.

It is easy to use ->> if we set >=triangle 90 first.

An example:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}

\tikz \draw[>=triangle 90, ->>]
  (0,0) -- (2,0);

\end{document}