\documentclass[preview]{standalone}
\usepackage{mwe}
\usepackage{subcaption}

\begin{document}

\def\FigSize{.33}
\begin{figure}
\begin{minipage}{2\FigSize \linewidth}
  \begin{subfigure}[b]{\FigSize \textwidth}
  \centering
    \includegraphics[width=\FigSize \textwidth]{example-image-a}
  \phantomcaption
  \end{subfigure}
  \begin{subfigure}[b]{\FigSize \textwidth}
  \centering
    \includegraphics[width=\FigSize \textwidth]{example-image-b}
  \phantomcaption
  \end{subfigure}
\end{minipage}
\end{figure}

\end{document}