Displaying numbers above each other (like a fraction), but without horizontal line and with less vertical space between them

Question

I want to have a 2x2 matrix of arrows (\uparrow and \downarrow) to illustrate electronic spins. I've managed to get a fraction without the horizontal line using \genfrac, but I can't figure out how to reduce the vertical spacing between the pairs of arrows down.

What I have so far (working example):

\documentclass[a4paper,10pt]{article}  
\usepackage{amsmath}  
\newcommand{\stack}[2]{\genfrac{}{}{0pt}{0}{#1}{#2}}  
\begin{document}  
$\stack{\uparrow\uparrow}{\downarrow\downarrow}$  
\end{document}

EDIT: This is what I ended up doing, after combining some of the answers here:

\newcommand{\stack}[2]{\array{c}{\scriptstyle #1}\\[-1.1ex]{\scriptstyle #2}\endarray}

And the final result, which is going into my report:

\documentclass[a4paper,10pt]{article}
\usepackage{amsmath}
\newcommand{\up}{\uparrow}
\newcommand{\dw}{\downarrow}
\newcommand{\stack}[2]{\array{c}{\scriptstyle #1}\\[-1.1ex]{\scriptstyle #2}\endarray}
\begin{document}

\begin{table}
    \centering
    \begin{tabular}{|c|c|c|c|c|}
        \hline
        Spin arrangements &Spins up &Degeneracy &Energy &Magnetization \\ \hline
        $\stack{\up\up}{\up\up}$ &4 &1 &-8J &4 \\
        $\stack{\up\up}{\up\dw}$ $\stack{\up\up}{\dw\up}$ $\stack{\up\dw}{\up\up}$ $\stack{\dw\up}{\up\up}$ &3 &4 &0 &2 \\
        $\stack{\up\up}{\dw\dw}$ $\stack{\dw\dw}{\up\up}$ $\stack{\up\dw}{\up\dw}$ $\stack{\dw\up}{\dw\up}$ &2 &4 &0 &0 \\
        $\stack{\up\dw}{\dw\up}$ $\stack{\dw\up}{\up\dw}$ &2 &2 &8J &0 \\
        $\stack{\up\dw}{\dw\dw}$ $\stack{\dw\up}{\dw\dw}$ $\stack{\dw\dw}{\up\dw}$ $\stack{\dw\dw}{\dw\up}$ &1 &4 &0 &-2 \\
        $\stack{\dw\dw}{\dw\dw}$ &0 &1 &-8J &-4 \\
        \hline
    \end{tabular}
\end{table}

\end{document}

Answer 1

try

$\array{c}\uparrow\uparrow\\\downarrow\downarrow\endarray$  

You can use it in a macro as well.