\documentclass{article}% Herbert Voss <voss@perce.de>
\usepackage{amsmath}
\makeatletter
\newcommand{\xLeftarrow}[2][]{%
    \ext@arrow 0055{\Leftarrowfill@}{#1}{#2}%
}
\def\Leftarrowfill@{\arrowfill@\Leftarrow\Relbar\Relbar}
\newcommand{\xRightarrow}[2][]{%
    \ext@arrow 0055{\Rightarrowfill@}{#1}{#2}%
}
\def\Rightarrowfill@{\arrowfill@\Relbar\Relbar\Rightarrow}
\makeatother
\begin{document}
	$\xRightarrow[under]{over}$
	$\xLeftarrow[under]{over}$
\end{document} 