documentclass{article}
newcommandmyatop[2]{left[{{#1}atop#2}right]} % "wrapper macro"
begin{document}

%% plain-TeX syntax
$ left[ {u atop v} right]$, $displaystyle left[ {u atop v} right]$

bigskip
%% LaTeX-type syntax
$ myatop{u}{v}$, $displaystyle myatop{u}{v}$

end{document}