\documentclass[12pt]{article}
\usepackage{fancyhdr}
\pagestyle{fancy}
\newcommand\firstL{2pt}
\newcommand\Abstand{2pt}
\makeatletter
\def\headrule{{\if@fancyplain\let\headrulewidth\plainheadrulewidth\fi
    \hrule\@height\firstL\@width\headwidth \vskip\Abstand%
    \hrule\@height\headrulewidth\@width\headwidth \vskip-\headrulewidth}}%
\makeatother

\begin{document}
\section{foo}
foo bar baz
\newpage
foo bar baz

\end{document}